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Fix CPU FlashAttention disabled on Linux/aarch64 by detecting L2 cache via cpuinfo #29621

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Fix CPU FlashAttention disabled on Linux/aarch64 by detecting L2 cache via cpuinfo #29621

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047ab5c
Fix CPU FlashAttention disabled on Linux/aarch64 by detecting L2 cach…
Sammy-Dabbas Jul 8, 2026
187fa19
Prefer sysconf L2 cache size and fall back to cpuinfo only when it fails
Sammy-Dabbas Jul 10, 2026
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21 changes: 20 additions & 1 deletion onnxruntime/core/platform/posix/env.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -303,7 +303,26 @@ class PosixEnv : public Env {

int GetL2CacheSize() const override {
#ifdef _SC_LEVEL2_CACHE_SIZE
return static_cast<int>(sysconf(_SC_LEVEL2_CACHE_SIZE));
// Prefer sysconf where it is implemented (e.g. Linux/x86), so those
// platforms keep their existing, working values. glibc's aarch64 backend
// does not implement the cache-size queries, so sysconf(_SC_LEVEL2_CACHE_SIZE)
// returns 0 on Linux/aarch64, which silently disables cache-tiled kernels
// (e.g. CPU FlashAttention in MultiHeadAttention). Only when sysconf reports
// no L2 do we fall back to cpuinfo, which reads the sizes from sysfs
// cacheinfo and works across architectures.
const auto l2_cache_size = sysconf(_SC_LEVEL2_CACHE_SIZE);
if (l2_cache_size > 0) {
return narrow<int>(l2_cache_size);
}
#ifdef ORT_USE_CPUINFO
if (cpuinfo_available_ && cpuinfo_get_l2_caches_count() > 0) {
const auto* l2_cache = cpuinfo_get_l2_cache(0);
if (l2_cache != nullptr && l2_cache->size > 0) {
return narrow<int>(l2_cache->size);
}
}
#endif // ORT_USE_CPUINFO
return narrow<int>(l2_cache_size);
#else
int value = 0; // unknown
#if (defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__)) && defined(HW_L2CACHESIZE)
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