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X-Bogosity: Ham, tests=bogofilter, spamicity=0.000005, 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 4/7/2025 11:31 PM, 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/pagemap") > c2f6ea38fc ("mm: page_alloc: don't steal single pages from biggest buddy") > acc4d5ff0b ("Merge tag 'net-6.15-rc0' of git://git.kernel.org/pub/scm/linux/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 ± 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 biggest buddy") > Closes: https://lore.kernel.org/oe-lkp/202503271547.fc08b188-lkp@intel.com > Cc: stable@vger.kernel.org # 6.10+ > Signed-off-by: Johannes Weiner > --- Tested on AMD Zen 3 EPYC (2-socket and 1 NUMA node, 64 CPUs on each socket) vm-scalability/300s-lru-file-mmap-read. Vanilla Patched Change Throughput 32267451 36112127 +11.9% improvement Stddev% 2477.36 2969.18 +19.8% Free Time 0.144072 0.148774 +3.2% Median 227967 249851 +9.6% Tested-by: Shivank Garg Thanks, Shivank