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selftests: vDSO: align getrandom states to cache line

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This prevents false sharing, which makes a large difference on machines
with several NUMA nodes, such as on a dual socket Intel(R) Xeon(R) Gold
6338 CPU @ 2.00GHz, where the "bench-multi" test goes from 2.7s down to
1.9s. While this is just test code, it also forms the basis of how folks
will wind up implementing this in libraries, so we should implement this
simple cache alignment improvement here.

Suggested-by: Florian Weimer <fweimer@redhat.com>
Cc: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
This commit is contained in:
Jason A. Donenfeld 2024-09-29 04:55:49 +02:00 committed by Shuah Khan
parent 45a8897db6
commit a18c835779
1 changed files with 5 additions and 3 deletions
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8
tools/testing/selftests/vDSO/vdso_test_getrandom.c
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@ -59,10 +59,12 @@ static void *vgetrandom_get_state(void)
size_t page_size = getpagesize();
size_t new_cap;
size_t alloc_size, num = sysconf(_SC_NPROCESSORS_ONLN); /* Just a decent heuristic. */
size_t state_size_aligned, cache_line_size = sysconf(_SC_LEVEL1_DCACHE_LINESIZE) ?: 1;
void *new_block, *new_states;
alloc_size = (num * vgrnd.params.size_of_opaque_state + page_size - 1) & (~(page_size - 1));
num = (page_size / vgrnd.params.size_of_opaque_state) * (alloc_size / page_size);
state_size_aligned = (vgrnd.params.size_of_opaque_state + cache_line_size - 1) & (~(cache_line_size - 1));
alloc_size = (num * state_size_aligned + page_size - 1) & (~(page_size - 1));
num = (page_size / state_size_aligned) * (alloc_size / page_size);
new_block = mmap(0, alloc_size, vgrnd.params.mmap_prot, vgrnd.params.mmap_flags, -1, 0);
if (new_block == MAP_FAILED)
goto out;
@ -78,7 +80,7 @@ static void *vgetrandom_get_state(void)
if (((uintptr_t)new_block & (page_size - 1)) + vgrnd.params.size_of_opaque_state > page_size)
new_block = (void *)(((uintptr_t)new_block + page_size - 1) & (~(page_size - 1)));
vgrnd.states[i] = new_block;
new_block += vgrnd.params.size_of_opaque_state;
new_block += state_size_aligned;
}
vgrnd.len = num;
goto success;