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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> Precedence: bulk X-Mailing-List: cgroups@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, 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.