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Wed, 19 Mar 2025 12:16:42 -0700 (PDT) Date: Wed, 19 Mar 2025 15:16:38 -0400 From: Johannes Weiner To: Yosry Ahmed Cc: Greg Thelen , Tejun Heo , Michal =?iso-8859-1?Q?Koutn=FD?= , Andrew Morton , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Eric Dumazet Subject: Re: [PATCH] cgroup/rstat: avoid disabling irqs for O(num_cpu) Message-ID: <20250319191638.GD1876369@cmpxchg.org> References: <20250319071330.898763-1-gthelen@google.com> <20250319180643.GC1876369@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: btw8hycjwkzxrdafmqbiy5d8bjfc4qb1 X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: B843B1C000A X-Rspam-User: X-HE-Tag: 1742411803-292539 X-HE-Meta: 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 FVYJIE5Y 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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 Wed, Mar 19, 2025 at 06:35:02PM +0000, Yosry Ahmed wrote: > On Wed, Mar 19, 2025 at 02:06:43PM -0400, Johannes Weiner wrote: > > (btw, why do we not have any locking around the root stats in > > cgroup_base_stat_cputime_show()? There isn't anything preventing a > > reader from seeing all zeroes if another reader runs the memset() on > > cgrp->bstat, is there? Or double times...) > > (I think root_cgroup_cputime() operates on a stack allocated bstat, not > cgrp->bstat) That was the case until: commit b824766504e49f3fdcbb8c722e70996a78c3636e Author: Chen Ridong Date: Thu Jul 4 14:01:19 2024 +0000 cgroup/rstat: add force idle show helper Now it's doing this: void cgroup_base_stat_cputime_show(struct seq_file *seq) { struct cgroup *cgrp = seq_css(seq)->cgroup; if (cgroup_parent(cgrp)) { ... } else { /* cgrp->bstat of root is not actually used, reuse it */ root_cgroup_cputime(&cgrp->bstat); usage = cgrp->bstat.cputime.sum_exec_runtime; utime = cgrp->bstat.cputime.utime; stime = cgrp->bstat.cputime.stime; ntime = cgrp->bstat.ntime; } } and then: static void root_cgroup_cputime(struct cgroup_base_stat *bstat) { struct task_cputime *cputime = &bstat->cputime; int i; memset(bstat, 0, sizeof(*bstat)); /* various += on bstat and cputime members */ } So all readers are mucking with the root cgroup's bstat.