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Mon, 17 Aug 2026 21:58:43 +0800 Message-ID: Date: Mon, 17 Aug 2026 21:58:39 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Cc: husong@kylinos.cn, akpm@linux-foundation.org, audra@redhat.com, bingfangguo@tencent.com, cgroups@vger.kernel.org, hannes@cmpxchg.org, joshua.hahnjy@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, muchun.song@linux.dev, roman.gushchin@linux.dev, shakeel.butt@linux.dev, zhuhui@kylinos.cn Subject: Re: [PATCH] mm: memcg: flush empty per-cpu stock slots on memcg offlining To: Michal Hocko References: <20260817131221.44761-1-husong@kylinos.cn> From: Song Hu In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: 1384z79gikbcgaj7wyk4nycgar53jxkc X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 2572820008 X-HE-Tag: 1786975136-618574 X-HE-Meta: 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 wf7I+/i/ EWx8x4aDKCEp449CcGQ0HPt4ud3BaRXLdXpawsQBUIlVY++vxnu8p33Xd4dYv//fxVCUabqLVc3oQim0d1N+wBOen5bF6JTdh3xpmflQhjTVU6812ZtbIB6Dj/JdTogZ6X40Q2bOVML4jW3aMpdZqJJi0sA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi,Michal 在 2026/8/17 21:29, Michal Hocko 写道: > On Mon 17-08-26 21:12:21, Song Hu wrote: >> On Mon 17-08-26, Michal Hocko wrote: >>> Is there any specific reason why the memcg stays in the cache slot >>> without any pages? >> >> consume_stock() doesn't release the slot when nr_pages hits zero. It >> is kept for the next charge of the same task and only gets displaced >> by a charge under a different memcg or by CPU hotplug. The problem is >> that the offlining drain skips empty slots, so the css reference they >> hold is never dropped unless something unrelated displaces them. > > This doesn't answer my question, really, does it? Is there any good > reason for this implementation? Why do we need to drop references > remotely when we can do so when the last cached charge is consumed? > Fair enough. There is no strong reason. Keeping the slot populated after the last page is consumed only saves a css_get()/css_put() pair when the same memcg charges again on that CPU - a micro-optimization from the original single-slot implementation. Dropping the reference in consume_stock() when the slot empties is the better place. Empty slots stop existing, so the offlining drain has nothing left to miss and is_memcg_drain_needed() stays as it is. This also makes Joshua's concern about the full-stock drain go away entirely. The cost is one refcount pair per emptied slot, at most once per MEMCG_CHARGE_BATCH pages. Joshua, this supersedes the css_is_dying gating you suggested and that I said I would do - with no empty slots left, the check would never fire, and the kill_css_sync() ordering argument becomes moot as well. Since you are reworking this code, I'd appreciate a sanity check on releasing from consume_stock(). If this direction works for you both, I'll rework the patch accordingly. Thanks, Song