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 0au26Yu/ 8QDBBt5QI/0XAHLR0L/cPYq6UPBoUCmqJ9OXmeR+tnmgi6Ml8l9pM3BtqeFVgQmvdQ0VQtLBAAg0PP4AlaP3zt7EN0/95EmA7BZKJOacUOoVdGxZc5AX/X6QFVM5S/hRB/JyPMpw604eTpFtv3ZT7hqjpv+zrxk3bx0jRmwuRXPaHnC2BbzqE4kEyMRGocmFfGX9ENbSzgfoB4528oA49DtiS0+/pjlqqa1yPd2TVkBqzVPD+d9STVCiEIHQI8TLuIb83i5MtoFWyzLAh5CSmbTXqeujQN3Jw1WrnAI5yux445S/X6IMnZP2hNPn2WbH4Q9b0X7kanCQPUrKUTo2XZA2K9olv8z8DSvwIsTCLeus/fwOaQxqiLmDwSnzbWSkPQl4Rg/loYZwnXQTbnBmzJMhQg9W1Gxuy1HH5XkYXg28b+ME1IYvUlqeSbwqr7U4+sHyM8rne3XjPv6vEnce1c36rxGl+R6s+KmWjos3cQe7wRKEc7BJs0QB+uw4vc5RSyJcfb9g+E/EovsAUBX655TkfF2e/nm0u8h8ALKMm28/1inRSvT90L1RQMrs0iC1fRNOb9KnLEHYOb0pjmW+6Ojtmzgo8rsYuXozm15bW8IgHtSidogMxYwSlqGqmlqk2ZYw7qa+mgYw1XAPIFk98Xyg/6KLr3ichBwTMILNDwl62LV8= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 08/08/25 12:28 am, 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. > > 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 > --- > mm/mremap.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/mm/mremap.c b/mm/mremap.c > index 677a4d744df9..9afa8cd524f5 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -179,6 +179,10 @@ static int mremap_folio_pte_batch(struct vm_area_struct *vma, unsigned long addr > if (max_nr == 1) > return 1; > > + /* Avoid expensive folio lookup if we stand no chance of benefit. */ > + if (pte_batch_hint(ptep, pte) == 1) > + return 1; > + > folio = vm_normal_folio(vma, addr, pte); > if (!folio || !folio_test_large(folio)) > return 1; > -- > 2.50.1 Thanks for debugging this! Reviewed-by: Dev Jain