[v5,2/2] rtld: Avoid using up static TLS surplus for optimizations [BZ #25051]

  On some targets static TLS surplus area can be used opportunistically
for dynamically loaded modules such that the TLS access then becomes
faster (TLSDESC and powerpc TLS optimization). However we don't want
all surplus TLS to be used for this optimization because dynamically
loaded modules with initial-exec model TLS can only use surplus TLS.

The new contract for surplus static TLS use is:

- libc.so can have up to 192 bytes of IE TLS,
- other system libraries together can have up to 144 bytes of IE TLS.
- Some "optional" static TLS is available for opportunistic use.

The optional TLS is now tunable: rtld.optional_static_tls, so users
can directly affect the allocated static TLS size. (Note that module
unloading with dlclose does not reclaim static TLS. After the optional
TLS runs out, TLS access is no longer optimized to use static TLS.)

The default setting of rtld.optional_static_tls is 512 so the surplus
TLS is 3*192 + 4*144 + 512 = 1664 by default, the same as before.

Fixes BZ #25051.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.

---

v5:
- Split the patch into two: rtld.nns and tls optimization parts.
- Use rtld tunable namespace.
- Set tunable min val to 0.
- New test using dlmopen.
- Tests now have check for failing IE TLS dlopen.
- Updated the tunable documentation.
v4:
- Rebased and moved this log out of the commit message.
- Minor commit message wording changes.
v3:
- archived at
  https://sourceware.org/pipermail/libc-alpha/2020-March/111660.html
- Replace TLS_STATIC_SURPLUS with GLRO(dl_tls_static_surplus) and
  simplify related logic.
- In case of static linking, replace GL(dl_tls_static_size) with
  GLRO(dl_tls_static_surplus) in the code paths before the
  GL(dl_tls_static_size) value is actually computed.
- Update comments and the test code.
- Document the new tunables.
- Update description, mention static linking.
v2:
- Add dl.nns tunable.
- Add dl.optional_static_tls tunable.
- New surplus TLS usage contract that works reliably up to dl.nns
  namespaces.
---
 csu/libc-tls.c             |   3 +
 elf/Makefile               |  29 +++++++++-
 elf/dl-reloc.c             |  37 +++++++++---
 elf/dl-tls.c               |   9 +--
 elf/dl-tunables.list       |   5 ++
 elf/dynamic-link.h         |   5 +-
 elf/tst-tls-ie-dlmopen.c   | 114 +++++++++++++++++++++++++++++++++++++
 elf/tst-tls-ie-mod.h       |  40 +++++++++++++
 elf/tst-tls-ie-mod0.c      |   4 ++
 elf/tst-tls-ie-mod1.c      |   4 ++
 elf/tst-tls-ie-mod2.c      |   4 ++
 elf/tst-tls-ie-mod3.c      |   4 ++
 elf/tst-tls-ie-mod4.c      |   4 ++
 elf/tst-tls-ie-mod5.c      |   4 ++
 elf/tst-tls-ie-mod6.c      |   4 ++
 elf/tst-tls-ie.c           | 113 ++++++++++++++++++++++++++++++++++++
 manual/tunables.texi       |  17 ++++++
 sysdeps/generic/ldsodefs.h |   3 +
 18 files changed, 386 insertions(+), 17 deletions(-)
 create mode 100644 elf/tst-tls-ie-dlmopen.c
 create mode 100644 elf/tst-tls-ie-mod.h
 create mode 100644 elf/tst-tls-ie-mod0.c
 create mode 100644 elf/tst-tls-ie-mod1.c
 create mode 100644 elf/tst-tls-ie-mod2.c
 create mode 100644 elf/tst-tls-ie-mod3.c
 create mode 100644 elf/tst-tls-ie-mod4.c
 create mode 100644 elf/tst-tls-ie-mod5.c
 create mode 100644 elf/tst-tls-ie-mod6.c
 create mode 100644 elf/tst-tls-ie.c
  

On 6/22/20 12:21 PM, Szabolcs Nagy wrote:
> On some targets static TLS surplus area can be used opportunistically
> for dynamically loaded modules such that the TLS access then becomes
> faster (TLSDESC and powerpc TLS optimization). However we don't want
> all surplus TLS to be used for this optimization because dynamically
> loaded modules with initial-exec model TLS can only use surplus TLS.
> 
> The new contract for surplus static TLS use is:
> 
> - libc.so can have up to 192 bytes of IE TLS,
> - other system libraries together can have up to 144 bytes of IE TLS.
> - Some "optional" static TLS is available for opportunistic use.
> 
> The optional TLS is now tunable: rtld.optional_static_tls, so users
> can directly affect the allocated static TLS size. (Note that module
> unloading with dlclose does not reclaim static TLS. After the optional
> TLS runs out, TLS access is no longer optimized to use static TLS.)
> 
> The default setting of rtld.optional_static_tls is 512 so the surplus
> TLS is 3*192 + 4*144 + 512 = 1664 by default, the same as before.
> 
> Fixes BZ #25051.
> 
> Tested on aarch64-linux-gnu and x86_64-linux-gnu.

OK for mater.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

 
> ---
> 
> v5:
> - Split the patch into two: rtld.nns and tls optimization parts.
> - Use rtld tunable namespace.
> - Set tunable min val to 0.
> - New test using dlmopen.
> - Tests now have check for failing IE TLS dlopen.
> - Updated the tunable documentation.
> v4:
> - Rebased and moved this log out of the commit message.
> - Minor commit message wording changes.
> v3:
> - archived at
>   https://sourceware.org/pipermail/libc-alpha/2020-March/111660.html
> - Replace TLS_STATIC_SURPLUS with GLRO(dl_tls_static_surplus) and
>   simplify related logic.
> - In case of static linking, replace GL(dl_tls_static_size) with
>   GLRO(dl_tls_static_surplus) in the code paths before the
>   GL(dl_tls_static_size) value is actually computed.
> - Update comments and the test code.
> - Document the new tunables.
> - Update description, mention static linking.
> v2:
> - Add dl.nns tunable.
> - Add dl.optional_static_tls tunable.
> - New surplus TLS usage contract that works reliably up to dl.nns
>   namespaces.
> ---
>  csu/libc-tls.c             |   3 +
>  elf/Makefile               |  29 +++++++++-
>  elf/dl-reloc.c             |  37 +++++++++---
>  elf/dl-tls.c               |   9 +--
>  elf/dl-tunables.list       |   5 ++
>  elf/dynamic-link.h         |   5 +-
>  elf/tst-tls-ie-dlmopen.c   | 114 +++++++++++++++++++++++++++++++++++++
>  elf/tst-tls-ie-mod.h       |  40 +++++++++++++
>  elf/tst-tls-ie-mod0.c      |   4 ++
>  elf/tst-tls-ie-mod1.c      |   4 ++
>  elf/tst-tls-ie-mod2.c      |   4 ++
>  elf/tst-tls-ie-mod3.c      |   4 ++
>  elf/tst-tls-ie-mod4.c      |   4 ++
>  elf/tst-tls-ie-mod5.c      |   4 ++
>  elf/tst-tls-ie-mod6.c      |   4 ++
>  elf/tst-tls-ie.c           | 113 ++++++++++++++++++++++++++++++++++++
>  manual/tunables.texi       |  17 ++++++
>  sysdeps/generic/ldsodefs.h |   3 +
>  18 files changed, 386 insertions(+), 17 deletions(-)
>  create mode 100644 elf/tst-tls-ie-dlmopen.c
>  create mode 100644 elf/tst-tls-ie-mod.h
>  create mode 100644 elf/tst-tls-ie-mod0.c
>  create mode 100644 elf/tst-tls-ie-mod1.c
>  create mode 100644 elf/tst-tls-ie-mod2.c
>  create mode 100644 elf/tst-tls-ie-mod3.c
>  create mode 100644 elf/tst-tls-ie-mod4.c
>  create mode 100644 elf/tst-tls-ie-mod5.c
>  create mode 100644 elf/tst-tls-ie-mod6.c
>  create mode 100644 elf/tst-tls-ie.c
> 
> diff --git a/csu/libc-tls.c b/csu/libc-tls.c
> index e2603157e8..fb77cd94fa 100644
> --- a/csu/libc-tls.c
> +++ b/csu/libc-tls.c
> @@ -56,6 +56,9 @@ size_t _dl_tls_static_align;
>     loaded modules with IE-model TLS or for TLSDESC optimization.
>     See comments in elf/dl-tls.c where it is initialized.  */
>  size_t _dl_tls_static_surplus;
> +/* Remaining amount of static TLS that may be used for optimizing
> +   dynamic TLS access (e.g. with TLSDESC).  */
> +size_t _dl_tls_static_optional;

OK.

>  
>  /* Generation counter for the dtv.  */
>  size_t _dl_tls_generation;
> diff --git a/elf/Makefile b/elf/Makefile
> index 6fe1df90bb..5fadaec27c 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -204,7 +204,8 @@ tests += restest1 preloadtest loadfail multiload origtest resolvfail \
>  	 tst-dlopen-self tst-auditmany tst-initfinilazyfail tst-dlopenfail \
>  	 tst-dlopenfail-2 \
>  	 tst-filterobj tst-filterobj-dlopen tst-auxobj tst-auxobj-dlopen \
> -	 tst-audit14 tst-audit15 tst-audit16
> +	 tst-audit14 tst-audit15 tst-audit16 \
> +	 tst-tls-ie tst-tls-ie-dlmopen

OK.

>  #	 reldep9
>  tests-internal += loadtest unload unload2 circleload1 \
>  	 neededtest neededtest2 neededtest3 neededtest4 \
> @@ -317,7 +318,11 @@ modules-names = testobj1 testobj2 testobj3 testobj4 testobj5 testobj6 \
>  		tst-dlopenfailmod1 tst-dlopenfaillinkmod tst-dlopenfailmod2 \
>  		tst-dlopenfailmod3 tst-ldconfig-ld-mod \
>  		tst-filterobj-flt tst-filterobj-aux tst-filterobj-filtee \
> -		tst-auditlogmod-1 tst-auditlogmod-2 tst-auditlogmod-3
> +		tst-auditlogmod-1 tst-auditlogmod-2 tst-auditlogmod-3 \
> +		tst-tls-ie-mod0 tst-tls-ie-mod1 tst-tls-ie-mod2 \
> +		tst-tls-ie-mod3 tst-tls-ie-mod4 tst-tls-ie-mod5 \
> +		tst-tls-ie-mod6

OK.

> +
>  # Most modules build with _ISOMAC defined, but those filtered out
>  # depend on internal headers.
>  modules-names-tests = $(filter-out ifuncmod% tst-libc_dlvsym-dso tst-tlsmod%,\
> @@ -1748,3 +1753,23 @@ $(objpfx)tst-auxobj: $(objpfx)tst-filterobj-aux.so
>  $(objpfx)tst-auxobj-dlopen: $(libdl)
>  $(objpfx)tst-auxobj.out: $(objpfx)tst-filterobj-filtee.so
>  $(objpfx)tst-auxobj-dlopen.out: $(objpfx)tst-filterobj-filtee.so
> +
> +$(objpfx)tst-tls-ie: $(libdl) $(shared-thread-library)
> +$(objpfx)tst-tls-ie.out: \
> +  $(objpfx)tst-tls-ie-mod0.so \
> +  $(objpfx)tst-tls-ie-mod1.so \
> +  $(objpfx)tst-tls-ie-mod2.so \
> +  $(objpfx)tst-tls-ie-mod3.so \
> +  $(objpfx)tst-tls-ie-mod4.so \
> +  $(objpfx)tst-tls-ie-mod5.so \
> +  $(objpfx)tst-tls-ie-mod6.so

OK.

> +
> +$(objpfx)tst-tls-ie-dlmopen: $(libdl) $(shared-thread-library)
> +$(objpfx)tst-tls-ie-dlmopen.out: \
> +  $(objpfx)tst-tls-ie-mod0.so \
> +  $(objpfx)tst-tls-ie-mod1.so \
> +  $(objpfx)tst-tls-ie-mod2.so \
> +  $(objpfx)tst-tls-ie-mod3.so \
> +  $(objpfx)tst-tls-ie-mod4.so \
> +  $(objpfx)tst-tls-ie-mod5.so \
> +  $(objpfx)tst-tls-ie-mod6.so

OK.

> diff --git a/elf/dl-reloc.c b/elf/dl-reloc.c
> index ffcc84d396..6d32e49467 100644
> --- a/elf/dl-reloc.c
> +++ b/elf/dl-reloc.c
> @@ -39,13 +39,16 @@
>  /* We are trying to perform a static TLS relocation in MAP, but it was
>     dynamically loaded.  This can only work if there is enough surplus in
>     the static TLS area already allocated for each running thread.  If this
> -   object's TLS segment is too big to fit, we fail.  If it fits,
> -   we set MAP->l_tls_offset and return.
> -   This function intentionally does not return any value but signals error
> -   directly, as static TLS should be rare and code handling it should
> -   not be inlined as much as possible.  */
> +   object's TLS segment is too big to fit, we fail with -1.  If it fits,
> +   we set MAP->l_tls_offset and return 0.
> +   A portion of the surplus static TLS can be optionally used to optimize
> +   dynamic TLS access (with TLSDESC or powerpc TLS optimizations).
> +   If OPTIONAL is true then TLS is allocated for such optimization and
> +   the caller must have a fallback in case the optional portion of surplus
> +   TLS runs out.  If OPTIONAL is false then the entire surplus TLS area is
> +   considered and the allocation only fails if that runs out.  */
>  int
> -_dl_try_allocate_static_tls (struct link_map *map)
> +_dl_try_allocate_static_tls (struct link_map *map, bool optional)

OK.

>  {
>    /* If we've already used the variable with dynamic access, or if the
>       alignment requirements are too high, fail.  */
> @@ -68,8 +71,14 @@ _dl_try_allocate_static_tls (struct link_map *map)
>  
>    size_t n = (freebytes - blsize) / map->l_tls_align;
>  
> -  size_t offset = GL(dl_tls_static_used) + (freebytes - n * map->l_tls_align
> -					    - map->l_tls_firstbyte_offset);
> +  /* Account optional static TLS surplus usage.  */
> +  size_t use = freebytes - n * map->l_tls_align - map->l_tls_firstbyte_offset;
> +  if (optional && use > GL(dl_tls_static_optional))
> +    goto fail;
> +  else if (optional)
> +    GL(dl_tls_static_optional) -= use;
> +
> +  size_t offset = GL(dl_tls_static_used) + use;
>  
>    map->l_tls_offset = GL(dl_tls_static_used) = offset;
>  #elif TLS_DTV_AT_TP
> @@ -83,6 +92,13 @@ _dl_try_allocate_static_tls (struct link_map *map)
>    if (used > GL(dl_tls_static_size))
>      goto fail;
>  
> +  /* Account optional static TLS surplus usage.  */
> +  size_t use = used - GL(dl_tls_static_used);
> +  if (optional && use > GL(dl_tls_static_optional))
> +    goto fail;
> +  else if (optional)
> +    GL(dl_tls_static_optional) -= use;
> +
>    map->l_tls_offset = offset;
>    map->l_tls_firstbyte_offset = GL(dl_tls_static_used);
>    GL(dl_tls_static_used) = used;
> @@ -110,12 +126,15 @@ _dl_try_allocate_static_tls (struct link_map *map)
>    return 0;
>  }
>  
> +/* This function intentionally does not return any value but signals error
> +   directly, as static TLS should be rare and code handling it should
> +   not be inlined as much as possible.  */
>  void
>  __attribute_noinline__
>  _dl_allocate_static_tls (struct link_map *map)
>  {
>    if (map->l_tls_offset == FORCED_DYNAMIC_TLS_OFFSET
> -      || _dl_try_allocate_static_tls (map))
> +      || _dl_try_allocate_static_tls (map, false))
>      {
>        _dl_signal_error (0, map->l_name, NULL, N_("\
>  cannot allocate memory in static TLS block"));
> diff --git a/elf/dl-tls.c b/elf/dl-tls.c
> index 2201a1cc1d..af5db12d08 100644
> --- a/elf/dl-tls.c
> +++ b/elf/dl-tls.c
> @@ -57,25 +57,26 @@
>     This should be large enough to cover runtime libraries of the
>     compiler such as libgomp and libraries in libc other than libc.so.  */
>  #define OTHER_IE_TLS 144
> -/* Size of additional surplus TLS, placeholder for TLS optimizations.  */
> -#define OPT_SURPLUS_TLS 512
>  
>  void
>  _dl_tls_static_surplus_init (void)
>  {
> -  size_t nns;
> +  size_t nns, opt_tls;
>  
>  #if HAVE_TUNABLES
>    nns = TUNABLE_GET (nns, size_t, NULL);
> +  opt_tls = TUNABLE_GET (optional_static_tls, size_t, NULL);
>  #else
>    /* Default values of the tunables.  */
>    nns = 4;
> +  opt_tls = 512;
>  #endif
>    if (nns > DL_NNS)
>      nns = DL_NNS;
> +  GL(dl_tls_static_optional) = opt_tls;
>    GLRO(dl_tls_static_surplus) = ((nns - 1) * LIBC_IE_TLS
>  				 + nns * OTHER_IE_TLS
> -				 + OPT_SURPLUS_TLS);
> +				 + opt_tls);

OK.

>  }
>  
>  /* Out-of-memory handler.  */
> diff --git a/elf/dl-tunables.list b/elf/dl-tunables.list
> index b07742d7b3..35634ef24d 100644
> --- a/elf/dl-tunables.list
> +++ b/elf/dl-tunables.list
> @@ -134,5 +134,10 @@ glibc {
>        maxval: 16
>        default: 4
>      }
> +    optional_static_tls {
> +      type: SIZE_T
> +      minval: 0
> +      default: 512

OK.

> +    }
>    }
>  }
> diff --git a/elf/dynamic-link.h b/elf/dynamic-link.h
> index bb7a66f4cd..6727233e1a 100644
> --- a/elf/dynamic-link.h
> +++ b/elf/dynamic-link.h
> @@ -40,9 +40,10 @@
>      (__builtin_expect ((sym_map)->l_tls_offset				\
>  		       != FORCED_DYNAMIC_TLS_OFFSET, 1)			\
>       && (__builtin_expect ((sym_map)->l_tls_offset != NO_TLS_OFFSET, 1)	\
> -	 || _dl_try_allocate_static_tls (sym_map) == 0))
> +	 || _dl_try_allocate_static_tls (sym_map, true) == 0))
>  
> -int _dl_try_allocate_static_tls (struct link_map *map) attribute_hidden;
> +int _dl_try_allocate_static_tls (struct link_map *map, bool optional)
> +  attribute_hidden;
>  
>  #include <elf.h>
>  
> diff --git a/elf/tst-tls-ie-dlmopen.c b/elf/tst-tls-ie-dlmopen.c
> new file mode 100644
> index 0000000000..0be47c7237
> --- /dev/null
> +++ b/elf/tst-tls-ie-dlmopen.c
> @@ -0,0 +1,114 @@
> +/* Test dlopen of modules with initial-exec TLS after dlmopen.
> +   Copyright (C) 2016-2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +/* This test tries to check that surplus static TLS is not used up for
> +   dynamic TLS optimizations and 4*144 = 576 bytes of static TLS is
> +   still available for dlopening modules with initial-exec TLS after 3
> +   new dlmopen namespaces are created.  It depends on rtld.nns=4 and
> +   rtld.optional_static_tls=512 tunable settings.  */

OK. Good comment.

> +
> +#include <errno.h>
> +#include <pthread.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +
> +static int do_test (void);
> +#include <support/xthread.h>
> +#include <support/xdlfcn.h>
> +#include <support/test-driver.c>
> +
> +/* Have some big TLS in the main exe: should not use surplus TLS.  */
> +__thread char maintls[1000];
> +
> +static pthread_barrier_t barrier;
> +
> +/* Forces multi-threaded behaviour.  */
> +static void *
> +blocked_thread_func (void *closure)
> +{
> +  xpthread_barrier_wait (&barrier);
> +  /* TLS load and access tests run here in the main thread.  */
> +  xpthread_barrier_wait (&barrier);
> +  return NULL;
> +}
> +
> +static void *
> +load_and_access (Lmid_t lmid, const char *mod, const char *func)
> +{
> +  /* Load module with TLS.  */
> +  void *p = xdlmopen (lmid, mod, RTLD_NOW);
> +  /* Access the TLS variable to ensure it is allocated.  */
> +  void (*f) (void) = (void (*) (void))xdlsym (p, func);
> +  f ();
> +  return p;
> +}
> +
> +static int
> +do_test (void)
> +{
> +  void *mods[5];
> +
> +  {
> +    int ret = pthread_barrier_init (&barrier, NULL, 2);
> +    if (ret != 0)
> +      {
> +        errno = ret;
> +        printf ("error: pthread_barrier_init: %m\n");
> +        exit (1);
> +      }
> +  }
> +
> +  pthread_t blocked_thread = xpthread_create (NULL, blocked_thread_func, NULL);
> +  xpthread_barrier_wait (&barrier);
> +
> +  printf ("maintls[%zu]:\t %p .. %p\n",
> +	   sizeof maintls, maintls, maintls + sizeof maintls);
> +  memset (maintls, 1, sizeof maintls);
> +
> +  /* Load modules with dynamic TLS (use surplus static TLS for libc
> +     in new namespaces and may be for TLS optimizations too).  */
> +  mods[0] = load_and_access (LM_ID_BASE, "tst-tls-ie-mod0.so", "access0");
> +  mods[1] = load_and_access (LM_ID_NEWLM, "tst-tls-ie-mod1.so", "access1");
> +  mods[2] = load_and_access (LM_ID_NEWLM, "tst-tls-ie-mod2.so", "access2");
> +  mods[3] = load_and_access (LM_ID_NEWLM, "tst-tls-ie-mod3.so", "access3");
> +  /* Load modules with initial-exec TLS (can only use surplus static TLS).  */
> +  mods[4] = load_and_access (LM_ID_BASE, "tst-tls-ie-mod6.so", "access6");
> +
> +  /* Here 576 bytes + 3 * libc use of surplus static TLS is in use so less
> +     than 1024 bytes are available (exact number depends on TLS optimizations
> +     and the libc TLS use).  */
> +  printf ("The next dlmopen should fail...\n");
> +  void *p = dlmopen (LM_ID_BASE, "tst-tls-ie-mod4.so", RTLD_NOW);
> +  if (p != NULL)
> +    {
> +      printf ("error: expected dlmopen to fail because there is "
> +	      "not enough surplus static TLS.\n");
> +      exit (1);

OK. You could have used FAIL_EXIT1 to merge both.

> +    }
> +  printf ("...OK failed with: %s.\n", dlerror ());
> +
> +  xpthread_barrier_wait (&barrier);
> +  xpthread_join (blocked_thread);
> +
> +  /* Close the modules.  */
> +  for (int i = 0; i < 5; ++i)
> +    xdlclose (mods[i]);
> +
> +  return 0;
> +}
> diff --git a/elf/tst-tls-ie-mod.h b/elf/tst-tls-ie-mod.h
> new file mode 100644
> index 0000000000..46b362a9b7
> --- /dev/null
> +++ b/elf/tst-tls-ie-mod.h
> @@ -0,0 +1,40 @@
> +/* Module with specified TLS size and model.
> +   Copyright (C) 2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +/* This file is parameterized by macros N, SIZE and MODEL.  */
> +
> +#include <stdio.h>
> +#include <string.h>
> +
> +#define CONCATX(x, y) x ## y
> +#define CONCAT(x, y) CONCATX (x, y)
> +#define STRX(x) #x
> +#define STR(x) STRX (x)
> +
> +#define VAR CONCAT (var, N)
> +
> +__attribute__ ((aligned (8), tls_model (MODEL)))
> +__thread char VAR[SIZE];
> +
> +void
> +CONCAT (access, N) (void)
> +{
> +  printf (STR (VAR) "[%d]:\t %p .. %p " MODEL "\n", SIZE, VAR, VAR + SIZE);
> +  fflush (stdout);
> +  memset (VAR, 1, SIZE);
> +}
> diff --git a/elf/tst-tls-ie-mod0.c b/elf/tst-tls-ie-mod0.c
> new file mode 100644
> index 0000000000..2450686e40
> --- /dev/null
> +++ b/elf/tst-tls-ie-mod0.c
> @@ -0,0 +1,4 @@
> +#define N 0
> +#define SIZE 480
> +#define MODEL "global-dynamic"
> +#include "tst-tls-ie-mod.h"
> diff --git a/elf/tst-tls-ie-mod1.c b/elf/tst-tls-ie-mod1.c
> new file mode 100644
> index 0000000000..849ff91e53
> --- /dev/null
> +++ b/elf/tst-tls-ie-mod1.c
> @@ -0,0 +1,4 @@
> +#define N 1
> +#define SIZE 120
> +#define MODEL "global-dynamic"
> +#include "tst-tls-ie-mod.h"
> diff --git a/elf/tst-tls-ie-mod2.c b/elf/tst-tls-ie-mod2.c
> new file mode 100644
> index 0000000000..23915ab67b
> --- /dev/null
> +++ b/elf/tst-tls-ie-mod2.c
> @@ -0,0 +1,4 @@
> +#define N 2
> +#define SIZE 24
> +#define MODEL "global-dynamic"
> +#include "tst-tls-ie-mod.h"
> diff --git a/elf/tst-tls-ie-mod3.c b/elf/tst-tls-ie-mod3.c
> new file mode 100644
> index 0000000000..5395f844a5
> --- /dev/null
> +++ b/elf/tst-tls-ie-mod3.c
> @@ -0,0 +1,4 @@
> +#define N 3
> +#define SIZE 16
> +#define MODEL "global-dynamic"
> +#include "tst-tls-ie-mod.h"
> diff --git a/elf/tst-tls-ie-mod4.c b/elf/tst-tls-ie-mod4.c
> new file mode 100644
> index 0000000000..93ac2eacae
> --- /dev/null
> +++ b/elf/tst-tls-ie-mod4.c
> @@ -0,0 +1,4 @@
> +#define N 4
> +#define SIZE 1024
> +#define MODEL "initial-exec"
> +#include "tst-tls-ie-mod.h"
> diff --git a/elf/tst-tls-ie-mod5.c b/elf/tst-tls-ie-mod5.c
> new file mode 100644
> index 0000000000..84b3fd285b
> --- /dev/null
> +++ b/elf/tst-tls-ie-mod5.c
> @@ -0,0 +1,4 @@
> +#define N 5
> +#define SIZE 128
> +#define MODEL "initial-exec"
> +#include "tst-tls-ie-mod.h"
> diff --git a/elf/tst-tls-ie-mod6.c b/elf/tst-tls-ie-mod6.c
> new file mode 100644
> index 0000000000..c736bf0684
> --- /dev/null
> +++ b/elf/tst-tls-ie-mod6.c
> @@ -0,0 +1,4 @@
> +#define N 6
> +#define SIZE 576
> +#define MODEL "initial-exec"
> +#include "tst-tls-ie-mod.h"
> diff --git a/elf/tst-tls-ie.c b/elf/tst-tls-ie.c
> new file mode 100644
> index 0000000000..c06454c50c
> --- /dev/null
> +++ b/elf/tst-tls-ie.c
> @@ -0,0 +1,113 @@
> +/* Test dlopen of modules with initial-exec TLS.
> +   Copyright (C) 2016-2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +/* This test tries to check that surplus static TLS is not used up for
> +   dynamic TLS optimizations and 3*192 + 4*144 = 1152 bytes of static
> +   TLS is available for dlopening modules with initial-exec TLS.  It
> +   depends on rtld.nns=4 and rtld.optional_static_tls=512 tunable setting.  */
> +
> +#include <errno.h>
> +#include <pthread.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +
> +static int do_test (void);
> +#include <support/xthread.h>
> +#include <support/xdlfcn.h>
> +#include <support/test-driver.c>
> +
> +/* Have some big TLS in the main exe: should not use surplus TLS.  */
> +__thread char maintls[1000];
> +
> +static pthread_barrier_t barrier;
> +
> +/* Forces multi-threaded behaviour.  */
> +static void *
> +blocked_thread_func (void *closure)
> +{
> +  xpthread_barrier_wait (&barrier);
> +  /* TLS load and access tests run here in the main thread.  */
> +  xpthread_barrier_wait (&barrier);
> +  return NULL;
> +}
> +
> +static void *
> +load_and_access (const char *mod, const char *func)
> +{
> +  /* Load module with TLS.  */
> +  void *p = xdlopen (mod, RTLD_NOW);
> +  /* Access the TLS variable to ensure it is allocated.  */
> +  void (*f) (void) = (void (*) (void))xdlsym (p, func);
> +  f ();
> +  return p;
> +}
> +
> +static int
> +do_test (void)
> +{
> +  void *mods[6];
> +
> +  {
> +    int ret = pthread_barrier_init (&barrier, NULL, 2);
> +    if (ret != 0)
> +      {
> +        errno = ret;
> +        printf ("error: pthread_barrier_init: %m\n");
> +        exit (1);
> +      }
> +  }
> +
> +  pthread_t blocked_thread = xpthread_create (NULL, blocked_thread_func, NULL);
> +  xpthread_barrier_wait (&barrier);
> +
> +  printf ("maintls[%zu]:\t %p .. %p\n",
> +	   sizeof maintls, maintls, maintls + sizeof maintls);
> +  memset (maintls, 1, sizeof maintls);
> +
> +  /* Load modules with dynamic TLS (may use surplus static TLS
> +     opportunistically).  */
> +  mods[0] = load_and_access ("tst-tls-ie-mod0.so", "access0");
> +  mods[1] = load_and_access ("tst-tls-ie-mod1.so", "access1");
> +  mods[2] = load_and_access ("tst-tls-ie-mod2.so", "access2");
> +  mods[3] = load_and_access ("tst-tls-ie-mod3.so", "access3");
> +  /* Load modules with initial-exec TLS (can only use surplus static TLS).  */
> +  mods[4] = load_and_access ("tst-tls-ie-mod4.so", "access4");
> +  mods[5] = load_and_access ("tst-tls-ie-mod5.so", "access5");
> +
> +  /* Here 1152 bytes of surplus static TLS is in use and at most 512 bytes
> +     are available (depending on TLS optimizations).  */
> +  printf ("The next dlopen should fail...\n");
> +  void *p = dlopen ("tst-tls-ie-mod6.so", RTLD_NOW);
> +  if (p != NULL)
> +    {
> +      printf ("error: expected dlopen to fail because there is "
> +	      "not enough surplus static TLS.\n");
> +      exit (1);

OK.

> +    }
> +  printf ("...OK failed with: %s.\n", dlerror ());
> +
> +  xpthread_barrier_wait (&barrier);
> +  xpthread_join (blocked_thread);
> +
> +  /* Close the modules.  */
> +  for (int i = 0; i < 6; ++i)
> +    xdlclose (mods[i]);
> +
> +  return 0;
> +}
> diff --git a/manual/tunables.texi b/manual/tunables.texi
> index 978e08f4fb..7f891c2710 100644
> --- a/manual/tunables.texi
> +++ b/manual/tunables.texi
> @@ -247,6 +247,23 @@ e.g. LD_AUDIT, or will use more than 4 dynamic link namespaces as created
>  by @code{dlmopen} with an lmid argument of @code{LM_ID_NEWLM}.
>  @end deftp
>  
> +@deftp Tunable glibc.rtld.optional_static_tls
> +Sets the amount of surplus static TLS in bytes to allocate at program
> +startup.  Every thread created allocates this amount of specified surplus
> +static TLS. This is a minimum value and additional space may be allocated
> +for internal purposes including alignment.  Optional static TLS is used for
> +optimizing dynamic TLS access for platforms that support such optimizations
> +e.g. TLS descriptors or optimized TLS access for POWER (@code{DT_PPC64_OPT}
> +and @code{DT_PPC_OPT}).  In order to make the best use of such optimizations
> +the value should be as many bytes as would be required to hold all TLS
> +variables in all dynamic loaded shared libraries.  The value cannot be known
> +by the dynamic loader because it doesn't know the expected set of shared
> +libraries which will be loaded.  The existing static TLS space cannot be
> +changed once allocated at process startup.  The default allocation of
> +optional static TLS is 512 bytes and is allocated in every thread.
> +@end deftp
> +
> +
>  @node Elision Tunables
>  @section Elision Tunables
>  @cindex elision tunables
> diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
> index 3b0c6d9620..997084fb4b 100644
> --- a/sysdeps/generic/ldsodefs.h
> +++ b/sysdeps/generic/ldsodefs.h
> @@ -442,6 +442,9 @@ struct rtld_global
>    EXTERN size_t _dl_tls_static_used;
>    /* Alignment requirement of the static TLS block.  */
>    EXTERN size_t _dl_tls_static_align;
> +  /* Remaining amount of static TLS that may be used for optimizing
> +     dynamic TLS access (e.g. with TLSDESC).  */
> +  EXTERN size_t _dl_tls_static_optional;
>  
>  /* Number of additional entries in the slotinfo array of each slotinfo
>     list element.  A large number makes it almost certain take we never
>