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Wed, 22 Mar 2023 21:29:57 -0700 (PDT) MIME-Version: 1.0 References: <20230323040037.2389095-1-yosryahmed@google.com> <20230323040037.2389095-2-yosryahmed@google.com> In-Reply-To: <20230323040037.2389095-2-yosryahmed@google.com> From: Shakeel Butt Date: Wed, 22 Mar 2023 21:29:46 -0700 Message-ID: Subject: Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the percpu lock To: Yosry Ahmed Cc: Tejun Heo , Josef Bacik , Jens Axboe , Zefan Li , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Andrew Morton , Vasily Averin , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, bpf@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: D2F9F120013 X-Rspam-User: X-Stat-Signature: fnt7tkjpqpx6p5qixr7p89ayidbnszfu X-HE-Tag: 1679545798-189792 X-HE-Meta: U2FsdGVkX1/dEt70AW15TgPGdgv47NSaRghtMeWzhQ2GvUz0sLM/JJg9NAU+V109eP9erp6ahGCv2Gc4sFB5N2Zwi+u5FMVad1sdXkC38i2NquHsRozIJPzpOqLq+rWTn7z7snJwvfiFsfk3XCCAWG5Gc+hGEAGpyX+fIroPN9SirvPZ+WGyye70OYQ1Jk4fdsel18gNzfxWPHhq/7XGWi2OjCx3qGKwBijtDudUXTbn2aJLoRSFpV/wdmyrcnERxt/jK/7Iz76vd77exC+58xDCdShME/2AOW97V1g5IDs+qL2LVH5CUjka2WUgaJu1DHqVkl4tk1Kp6ttWRCMkuTnC1rIJLyz64v15Y4Ih3P2wwsBb91GFuZlcEdAFS4b1djVF4TKfIMqwVMF531j+yk7fgaa6gFoXeNRTUugBjRTmakWUsU1lyy3rlBGWtZ9uIGvUqCWp/mhHyw3bIiF95bBKwX/I/NBCzP/ZrG2haEdtE6OMvfuOhyy+UYFvd/86Xep/jIZtliMQq9FIYkntAZ9fIiUX7AhDGLFfaxAcZrzIY3nMr04OdjGXt2HxUYFSPbuSXDz6CgP43MiWXJ3Mvg4QGhQEduhlJCw4+ABpu6K776uZY5DxeWgOhMmO7tkfI3ZJurd4x/Qqpf6xFUAyCFRumkGLbkGqCT710JfQ78fcEvvquVXQGfwxw5B7OkmIyExnOjq27rqzZLWT//hAQaafbbAtRB9cM5cOhbXF/vy1PFBQDrz6uSzSs3DF145GJPj0HlYwUo/X+nb7Up2pVQ9QOtz4qfoS7z2CkZREGMg+QXCmQD3q1Sa+i/M/stuC7Sro3clxNUt3nE7hB8sC8HJfbD90Elw7/752L5yLc9bUHtnAHMXwRc6YHHfmbqS9L6bE156uIIQSRQFUF45+il3jccpapLsfYY5k+GeV55SaaVZDFVWL1rNS+1ppXtHmYfErp7RdXhxdIOvII+M kg/8eOUg qn2mNPJVjT9afRme6KgeWbwEWO29nCV1DVOjJ3paB4omKbsuq7qhetLNhgfZWIXeuj5s+8Hw3CFw21St3OP6eQAO70yivg3zqKt+7vUrh32Sy8I+COZ0hEe5/DyAgwJ1L2efxI19PzGKkAp1awR9Y8SvUxhkO9k4uC1TvnGste0PlsT5EFvCZjtpZLvD91jopZ4dsQ4f1zA7vNUjK2LLC92TIRw3amyQ8DmqBZVqjJovKbkWbedXyCRHr9GNGWEBuq2tUhlepqMWAeyY= 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: On Wed, Mar 22, 2023 at 9:00=E2=80=AFPM Yosry Ahmed = wrote: > > Currently, when sleeping is not allowed during rstat flushing, we hold > the global rstat lock with interrupts disabled throughout the entire > flush operation. Flushing in an O(# cgroups * # cpus) operation, and > having interrupts disabled throughout is dangerous. > > For some contexts, we may not want to sleep, but can be interrupted > (e.g. while holding a spinlock or RCU read lock). As such, do not > disable interrupts throughout rstat flushing, only when holding the > percpu lock. This breaks down the O(# cgroups * # cpus) duration with > interrupts disabled to a series of O(# cgroups) durations. > > Furthermore, if a cpu spinning waiting for the global rstat lock, it > doesn't need to spin with interrupts disabled anymore. > > If the caller of rstat flushing needs interrupts to be disabled, it's up > to them to decide that, and it should be fine to hold the global rstat > lock with interrupts disabled. There is currently a single context that > may invoke rstat flushing with interrupts disabled, the > mem_cgroup_flush_stats() call in mem_cgroup_usage(), if called from > mem_cgroup_threshold(). > > To make it safe to hold the global rstat lock with interrupts enabled, > make sure we only flush from in_task() contexts. The side effect of that > we read stale stats in interrupt context, but this should be okay, as > flushing in interrupt context is dangerous anyway as it is an expensive > operation, so reading stale stats is safer. > > Signed-off-by: Yosry Ahmed Couple of questions: 1. What exactly is cgroup_rstat_lock protecting? Can we just remove it altogether? 2. Are we really calling rstat flush in irq context? 3. The mem_cgroup_flush_stats() call in mem_cgroup_usage() is only done for root memcg. Why is mem_cgroup_threshold() interested in root memcg usage? Why not ignore root memcg in mem_cgroup_threshold() ?