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Wed, 1 Apr 2026 11:01:52 -0400 (EDT) Date: Wed, 1 Apr 2026 16:01:51 +0100 From: Kiryl Shutsemau To: Breno Leitao Cc: Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, shakeel.butt@linux.dev, usama.arif@linux.dev, kernel-team@meta.com Subject: Re: [PATCH] mm/vmstat: spread vmstat_update requeue across the stat interval Message-ID: References: <20260401-vmstat-v1-1-b68ce4a35055@debian.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260401-vmstat-v1-1-b68ce4a35055@debian.org> X-Rspamd-Queue-Id: 0101AA000F X-Stat-Signature: dp84pcicri5g4e437nb16bkqi3c735hf X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1775055715-370067 X-HE-Meta: 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 8hhXyvyt qc/e+ZGtfNGG97JLUHyWfAWyzu9nJrRh2GPaapSJ2XEx3s9QOHdBke+yWMULrPGd4peOn5hXB0v81Zdx8kdo8HQ+A4w458QpGeyoSIV1mZ0OapsQoC2Mvi83WiAEygd8+RjPyTIh0t+/3ywS85VNw0k5YtzMng7GePyUsEeEv+tTP4k+DVaWtrz43Z7inJunIVrBCGT3+Zup6371xgKpFFSKIrlqDpNfLPfJ1SM+hH/ZHh7RgX7qlqfg1mYNb8l4Urauiob9KLQ8zRglzCVNWcbWYRvMZ2x19fceXW6oXHzGxzKVHGEgAe4SXQZ86QFXUE6Go7hg5EU+7+GLCJ94RGJNWpfF33OBvYG62ssnOKdU0F4NO4Ma4YucXPdTMTTVabFRP8pd//2T6kCU= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Apr 01, 2026 at 06:57:50AM -0700, Breno Leitao wrote: > vmstat_update uses round_jiffies_relative() when re-queuing itself, > which aligns all CPUs' timers to the same second boundary. When many > CPUs have pending PCP pages to drain, they all call decay_pcp_high() -> > free_pcppages_bulk() simultaneously, serializing on zone->lock and > hitting contention. > > Introduce vmstat_spread_delay() which distributes each CPU's > vmstat_update evenly across the stat interval instead of aligning them. Nice idea. > This does not increase the number of timer interrupts — each CPU still > fires once per interval. The timers are simply staggered rather than > aligned. Additionally, vmstat_work is DEFERRABLE_WORK, so it does not > wake idle CPUs regardless of scheduling; the spread only affects CPUs > that are already active > > `perf lock contention` shows 7.5x reduction in zone->lock contention > (872 -> 117 contentions, 199ms -> 81ms total wait) on a 72-CPU aarch64 > system under memory pressure. Wow. That's huge improvement. > > Tested on a 72-CPU aarch64 system using stress-ng --vm to generate > memory allocation bursts. Lock contention was measured with: > > perf lock contention -a -b -S free_pcppages_bulk > > Results with KASAN enabled: > > free_pcppages_bulk contention (KASAN): > +--------------+----------+----------+ > | Metric | No fix | With fix | > +--------------+----------+----------+ > | Contentions | 872 | 117 | > | Total wait | 199.43ms | 80.76ms | > | Max wait | 4.19ms | 35.76ms | > +--------------+----------+----------+ > > Results without KASAN: > > free_pcppages_bulk contention (no KASAN): > +--------------+----------+----------+ > | Metric | No fix | With fix | > +--------------+----------+----------+ > | Contentions | 240 | 133 | > | Total wait | 34.01ms | 24.61ms | > | Max wait | 965us | 1.35ms | > +--------------+----------+----------+ > > Signed-off-by: Breno Leitao Acked-by: Kiryl Shutsemau (Meta) -- Kiryl Shutsemau / Kirill A. Shutemov