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Tue, 16 Jul 2024 17:35:41 -0700 (PDT) MIME-Version: 1.0 References: <172070450139.2992819.13210624094367257881.stgit@firesoul> In-Reply-To: From: Yosry Ahmed Date: Tue, 16 Jul 2024 17:35:05 -0700 Message-ID: Subject: Re: [PATCH V7 1/2] cgroup/rstat: Avoid thundering herd problem by kswapd across NUMA nodes To: Jesper Dangaard Brouer Cc: tj@kernel.org, cgroups@vger.kernel.org, shakeel.butt@linux.dev, hannes@cmpxchg.org, lizefan.x@bytedance.com, longman@redhat.com, kernel-team@cloudflare.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam03 X-Rspam-User: X-Rspamd-Queue-Id: DDA69C0014 X-Stat-Signature: hfwxuoozznxxbz7nxtujajurcrwsh3kn X-HE-Tag: 1721176543-953416 X-HE-Meta: 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 TT0Zj9aS +UkEQ5fnm+4G2Lb1SX+A2IANjXYGtNayOTdivSv3CyPo0fTqVLkJ7txCSghh8lYyd4NtuB0muP8xC84c8NE0qMVoipEMEgdgKqKGKMPTQddnS9cakv8AbninH17yiM32SajdD X-Bogosity: Ham, tests=bogofilter, spamicity=0.237168, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: [..] > > > This is a clean (meaning no cadvisor interference) example of kswapd > starting simultaniously on many NUMA nodes, that in 27 out of 98 cases > hit the race (which is handled in V6 and V7). > > The BPF "cnt" maps are getting cleared every second, so this > approximates per sec numbers. This patch reduce pressure on the lock, > but we are still seeing (kfunc:vmlinux:cgroup_rstat_flush_locked) full > flushes approx 37 per sec (every 27 ms). On the positive side > ongoing_flusher mitigation stopped 98 per sec of these. > > In this clean kswapd case the patch removes the lock contention issue > for kswapd. The lock_contended cases 27 seems to be all related to > handled_race cases 27. > > The remaning high flush rate should also be addressed, and we should > also work on aproaches to limit this like my ealier proposal[1]. I honestly don't think a high number of flushes is a problem on its own as long as we are not spending too much time flushing, especially when we have magnitude-based thresholding so we know there is something to flush (although it may not be relevant to what we are doing). If we keep observing a lot of lock contention, one thing that I thought about is to have a variant of spin_lock with a timeout. This limits the flushing latency, instead of limiting the number of flushes (which I believe is the wrong metric to optimize). It also seems to me that we are doing a flush each 27ms, and your proposed threshold was once per 50ms. It doesn't seem like a fundamental difference. I am also wondering how many more flushes could be skipped if we handle the case of multiple ongoing flushers (whether by using a mutex, or making it a per-cgroup property as I suggested earlier).