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From: Shakeel Butt To: Jakub Kicinski Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Vlastimil Babka , Eric Dumazet , Soheil Hassas Yeganeh , linux-mm@kvack.org, cgroups@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Meta kernel team Subject: Re: [PATCH] memcg: multi-memcg percpu charge cache Message-ID: References: <20250416180229.2902751-1-shakeel.butt@linux.dev> <20250422181022.308116c1@kernel.org> <20250423153046.54d135f2@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250423153046.54d135f2@kernel.org> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: D4FED80010 X-Stat-Signature: titmwhffcja3miomz5qwcch59me6ks6y X-HE-Tag: 1745449165-577397 X-HE-Meta: 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 MXp1jevA JugRbYt+2qN3Tttb2weEQCro7nJC8cQHJ3kJdUCIxHRH1F6aYTAhvPGSlK2vIv3+iq4jVaF+M0+3eJKMkUbNqCmSoVk4pzidd3AU268xu8vhlRD/GTbY5sswUqaxcdiWzjtctRIS/fHIFZe+ElUdvZrqaiQW5pTkZjiqh9NNi3K6ViABmSJB321K0WQ== 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, Apr 23, 2025 at 03:30:46PM -0700, Jakub Kicinski wrote: > On Wed, 23 Apr 2025 15:16:56 -0700 Shakeel Butt wrote: > > > > - if (!local_trylock_irqsave(&memcg_stock.stock_lock, flags)) { > > > > + if (nr_pages > MEMCG_CHARGE_BATCH || > > > > + !local_trylock_irqsave(&memcg_stock.stock_lock, flags)) { > > > > /* > > > > - * In case of unlikely failure to lock percpu stock_lock > > > > - * uncharge memcg directly. > > > > + * In case of larger than batch refill or unlikely failure to > > > > + * lock the percpu stock_lock, uncharge memcg directly. > > > > */ > > > > > > We're bypassing the cache for > CHARGE_BATCH because the u8 math > > > may overflow? Could be useful to refocus the comment on the 'why' > > > > We actually never put more than MEMCG_CHARGE_BATCH in the cache and thus > > we can use u8 as type here. Though we may increase the batch size in > > future, so I should put a BUILD_BUG_ON somewhere here. > > No idea if this matters enough to deserve its own commit but basically > I was wondering if that behavior change is a separate optimization. > > Previously we'd check if the cache was for the releasing cgroup and sum > was over BATCH - drain its stock completely. Now we bypass looking at > the cache if nr_pages > BATCH so the cgroup may retain some stock. Yes indeed there is a little bit behavior change as you have explained. The older behavior (fully drain if nr_pages > BATCH) might be due to single per-cpu memcg cache limitation and in my opinion is problematic in some scenarios. If you see commit 5387c90490f7 ("mm/memcg: improve refill_obj_stock() performance"), a very similar behavior for objcg cache was having a performance impact and was optimized by only allowing the drain for some code paths. With multi-memcg support, I think we don't need to worry about it. Multi-objcg per-cpu cache is also on my TODO list.