[v1] x86: Fix slight bug in `shared_per_thread` cache size calculation.
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Commit Message
After:
```
commit af992e7abdc9049714da76cae1e5e18bc4838fb8
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date: Wed Jun 7 13:18:01 2023 -0500
x86: Increase `non_temporal_threshold` to roughly `sizeof_L3 / 4`
```
Split `shared` (cumulative cache size) from `shared_per_thread` (cache
size per socket), the `shared_per_thread` *can* be slightly off from
the previous calculation.
Previously we added `core` even if `threads_l2` was invalid, and only
used `threads_l2` to divide `core` if it was present. The changed
version only included `core` if `threads_l2` was valid.
This change restores the old behavior if `threads_l2` is invalid by
adding the entire value of `core`.
---
sysdeps/x86/dl-cacheinfo.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
Comments
On Mon, Jul 17, 2023 at 11:14 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> After:
> ```
> commit af992e7abdc9049714da76cae1e5e18bc4838fb8
> Author: Noah Goldstein <goldstein.w.n@gmail.com>
> Date: Wed Jun 7 13:18:01 2023 -0500
>
> x86: Increase `non_temporal_threshold` to roughly `sizeof_L3 / 4`
> ```
>
> Split `shared` (cumulative cache size) from `shared_per_thread` (cache
> size per socket), the `shared_per_thread` *can* be slightly off from
> the previous calculation.
>
> Previously we added `core` even if `threads_l2` was invalid, and only
> used `threads_l2` to divide `core` if it was present. The changed
> version only included `core` if `threads_l2` was valid.
>
> This change restores the old behavior if `threads_l2` is invalid by
> adding the entire value of `core`.
> ---
> sysdeps/x86/dl-cacheinfo.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
> index c98fa57a7b..43be2c1229 100644
> --- a/sysdeps/x86/dl-cacheinfo.h
> +++ b/sysdeps/x86/dl-cacheinfo.h
> @@ -614,8 +614,8 @@ get_common_cache_info (long int *shared_ptr, long int * shared_per_thread_ptr, u
> /* Account for non-inclusive L2 and L3 caches. */
> if (!inclusive_cache)
> {
> - if (threads_l2 > 0)
> - shared_per_thread += core / threads_l2;
> + long int core_per_thread = threads_l2 > 0 ? (core / threads_l2) : core;
> + shared_per_thread += core_per_thread;
> shared += core;
> }
>
> --
> 2.34.1
>
Noticed this when working on the AMD patch, but think the
fix should be seperate.
Noah Goldstein via Libc-alpha <libc-alpha@sourceware.org> writes:
> if (!inclusive_cache)
> {
> - if (threads_l2 > 0)
> - shared_per_thread += core / threads_l2;
> + long int core_per_thread = threads_l2 > 0 ? (core / threads_l2) : core;
> + shared_per_thread += core_per_thread;
> shared += core;
> }
So the original code read:
if (!inclusive_cache)
{
if (threads_l2 > 0)
core /= threads_l2;
shared += core;
}
And the new code reads:
if (!inclusive_cache)
{
long int core_per_thread = threads_l2 > 0 ? (core / threads_l2) : core;
shared_per_thread += core_per_thread;
shared += core;
}
"core_per_thread" is thus either all of core, or, if we have the data,
core/threads_l2. But, "core" doesn't change, so the value stored in
"shared" is different than way-back-when. However, the original change
also changed the temporal threshold from "shared" to
"shared_per_thread", which is initialized to "core". So the resulting
value used later should be effectively the same.
In the interim code:
if (!inclusive_cache)
{
if (threads_l2 > 0)
shared_per_thread += core / threads_l2;
shared += core;
}
the new "shared_per_thread" doesn't get the default value of "core" when
threads_l2 is zero.
This all seems right to me.
LGTM
Reviewed-by: DJ Delorie <dj@redhat.com>
@@ -614,8 +614,8 @@ get_common_cache_info (long int *shared_ptr, long int * shared_per_thread_ptr, u
/* Account for non-inclusive L2 and L3 caches. */
if (!inclusive_cache)
{
- if (threads_l2 > 0)
- shared_per_thread += core / threads_l2;
+ long int core_per_thread = threads_l2 > 0 ? (core / threads_l2) : core;
+ shared_per_thread += core_per_thread;
shared += core;
}