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 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 Mon Apr 7, 2025 at 2:01 PM EDT, Johannes Weiner wrote: > The test robot identified c2f6ea38fc1b ("mm: page_alloc: don't steal > single pages from biggest buddy") as the root cause of a 56.4% > regression in vm-scalability::lru-file-mmap-read. > > Carlos reports an earlier patch, c0cd6f557b90 ("mm: page_alloc: fix > freelist movement during block conversion"), as the root cause for a > regression in worst-case zone->lock+irqoff hold times. > > Both of these patches modify the page allocator's fallback path to be > less greedy in an effort to stave off fragmentation. The flip side of > this is that fallbacks are also less productive each time around, > which means the fallback search can run much more frequently. > > Carlos' traces point to rmqueue_bulk() specifically, which tries to > refill the percpu cache by allocating a large batch of pages in a > loop. It highlights how once the native freelists are exhausted, the > fallback code first scans orders top-down for whole blocks to claim, > then falls back to a bottom-up search for the smallest buddy to steal. > For the next batch page, it goes through the same thing again. > > This can be made more efficient. Since rmqueue_bulk() holds the > zone->lock over the entire batch, the freelists are not subject to > outside changes; when the search for a block to claim has already > failed, there is no point in trying again for the next page. > > Modify __rmqueue() to remember the last successful fallback mode, and > restart directly from there on the next rmqueue_bulk() iteration. > > Oliver confirms that this improves beyond the regression that the test > robot reported against c2f6ea38fc1b: > > commit: > f3b92176f4 ("tools/selftests: add guard region test for /proc/$pid/page= map") > c2f6ea38fc ("mm: page_alloc: don't steal single pages from biggest budd= y") > acc4d5ff0b ("Merge tag 'net-6.15-rc0' of git://git.kernel.org/pub/scm/l= inux/kernel/git/netdev/net") > 2c847f27c3 ("mm: page_alloc: speed up fallbacks in rmqueue_bulk()") <= --- your patch > > f3b92176f4f7100f c2f6ea38fc1b640aa7a2e155cc1 acc4d5ff0b61eb1715c498b6536 = 2c847f27c37da65a93d23c237c5 > ---------------- --------------------------- --------------------------- = --------------------------- > %stddev %change %stddev %change %stddev = %change %stddev > \ | \ | \ = | \ > 25525364 =C2=B1 3% -56.4% 11135467 -57.8% 10779336 = +31.6% 33581409 vm-scalability.throughput > > Carlos confirms that worst-case times are almost fully recovered > compared to before the earlier culprit patch: > > 2dd482ba627d (before freelist hygiene): 1ms > c0cd6f557b90 (after freelist hygiene): 90ms > next-20250319 (steal smallest buddy): 280ms > this patch : 8ms > > Reported-by: kernel test robot > Reported-by: Carlos Song > Tested-by: kernel test robot > Fixes: c0cd6f557b90 ("mm: page_alloc: fix freelist movement during block = conversion") > Fixes: c2f6ea38fc1b ("mm: page_alloc: don't steal single pages from bigge= st buddy") > Closes: https://lore.kernel.org/oe-lkp/202503271547.fc08b188-lkp@intel.co= m > Cc: stable@vger.kernel.org # 6.10+ > Signed-off-by: Johannes Weiner > --- > mm/page_alloc.c | 100 +++++++++++++++++++++++++++++++++++------------- > 1 file changed, 74 insertions(+), 26 deletions(-) > It is a really nice cleanup. It improves my understanding of the rmqueue*() and the whole flow a lot. Thank you for the patch. Acked-by: Zi Yan --=20 Best Regards, Yan, Zi