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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kas@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> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Wed, Apr 01, 2026 at 07:39:28AM -0700, Breno Leitao wrote: > Hello Johannes, > > On Wed, Apr 01, 2026 at 10:25:35AM -0400, Johannes Weiner wrote: > > On Wed, Apr 01, 2026 at 06:57:50AM -0700, Breno Leitao wrote: > > > +static unsigned long vmstat_spread_delay(void) > > > +{ > > > + unsigned long interval = sysctl_stat_interval; > > > + unsigned int nr_cpus = num_online_cpus(); > > > + > > > + if (nr_cpus <= 1) > > > + return round_jiffies_relative(interval); > > > + > > > + /* > > > + * Spread per-cpu vmstat work evenly across the interval. Don't > > > + * use round_jiffies_relative() here -- it would snap every CPU > > > + * back to the same second boundary, defeating the spread. > > > + */ > > > + return interval + (interval * (smp_processor_id() % nr_cpus)) / nr_cpus; > > > > smp_processor_id() <= nr_cpus, so > > > > return interval + interval*cpu/nr_cpus > > > > should be equivalent, no? > > nr_cpus is the number of online CPUs, while smp_processor_id() is the > CPU id. > > If you offline a CPU, then smp_processor_id() might be bigger than > num_online_cpus() > > My goal was to linearly shift the timer and avoid creating gaps when > removing certain CPUs. Ah makes sense. Plus you'd spill into the next interval otherwise.