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+0100, Pedro Falcato wrote: > On Thu, Aug 07, 2025 at 07:58:19PM +0100, Lorenzo Stoakes wrote: > > It was discovered in the attached report that commit f822a9a81a31 ("mm: > > optimize mremap() by PTE batching") introduced a significant performance > > regression on a number of metrics on x86-64, most notably > > stress-ng.bigheap.realloc_calls_per_sec - indicating a 37.3% regression in > > number of mremap() calls per second. > > > > I was able to reproduce this locally on an intel x86-64 raptor lake system, > > noting an average of 143,857 realloc calls/sec (with a stddev of 4,531 or > > 3.1%) prior to this patch being applied, and 81,503 afterwards (stddev of > > 2,131 or 2.6%) - a 43.3% regression. > > > > During testing I was able to determine that there was no meaningful > > difference in efforts to optimise the folio_pte_batch() operation, nor > > checking folio_test_large(). > > > > This is within expectation, as a regression this large is likely to > > indicate we are accessing memory that is not yet in a cache line (and > > perhaps may even cause a main memory fetch). > > > > The expectation by those discussing this from the start was that > > vm_normal_folio() (invoked by mremap_folio_pte_batch()) would likely be the > > culprit due to having to retrieve memory from the vmemmap (which mremap() > > page table moves does not otherwise do, meaning this is inevitably cold > > memory). > > > > I was able to definitively determine that this theory is indeed correct and > > the cause of the issue. > > > > The solution is to restore part of an approach previously discarded on > > review, that is to invoke pte_batch_hint() which explicitly determines, > > through reference to the PTE alone (thus no vmemmap lookup), what the PTE > > batch size may be. > > > > On platforms other than arm64 this is currently hardcoded to return 1, so > > this naturally resolves the issue for x86-64, and for arm64 introduces > > little to no overhead as the pte cache line will be hot. > > > > With this patch applied, we move from 81,503 realloc calls/sec to > > 138,701 (stddev of 496.1 or 0.4%), which is a -3.6% regression, however > > accounting for the variance in the original result, this is broadly > > restoring performance to its prior state. > > > > So, do we still have a regression then? If so, do we have any idea why? It's within 1 stddev of the original results, so I'd say it's possibly noise. Let's see what the bots say. If there's something else we can obviously take a look, I think Jann's point about cold cache is the key point here, and the delta here is indicative. > > > Reported-by: kernel test robot > > Closes: https://lore.kernel.org/oe-lkp/202508071609.4e743d7c-lkp@intel.com > > Fixes: f822a9a81a31 ("mm: optimize mremap() by PTE batching") > > Signed-off-by: Lorenzo Stoakes > > Fix looks great, thanks! > > Acked-by: Pedro Falcato Thanks! > > -- > Pedro Cheers, Lorenzo