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Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 Subject: Re: [PATCH] mm: memcg: provide accurate stats for userspace reads Content-Language: en-US To: Tejun Heo , Yosry Ahmed Cc: Michal Hocko , Shakeel Butt , Johannes Weiner , Roman Gushchin , Andrew Morton , Muchun Song , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 X-Rspamd-Queue-Id: 1D463C0017 X-Rspam-User: X-Stat-Signature: 9oouzoc5wu5137w1ghfjzpaaz37tsju4 X-Rspamd-Server: rspam01 X-HE-Tag: 1692114299-850795 X-HE-Meta: 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 oqizaOIA 2Q8g48F8fqXkPVWgPBRikNPmBtJoGDb8/EPln7ZqUWQ67UUFR9p6IRyuu4r11qAKkCQX18tkKpmsm6+S1+1Dbvoz7t0Cv5PzDlwDhHX3ULic4jUBQSnE8D9uiAm7PG+cA7y01xFbQ9Pf6UnV0ItTISjaLGKZwBwAOhI0c 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 8/14/23 20:35, Tejun Heo wrote: > Hello, > > On Mon, Aug 14, 2023 at 05:28:22PM -0700, Yosry Ahmed wrote: >>> So, the original design used mutex for synchronize flushing with the idea >>> being that updates are high freq but reads are low freq and can be >>> relatively slow. Using rstats for mm internal operations changed this >>> assumption quite a bit and we ended up switching that mutex with a lock. >> Naive question, do mutexes handle thundering herd problems better than >> spinlocks? I would assume so but I am not sure. > I don't know. We can ask Waiman if that becomes a problem. We had essentially solved the thundering herd problems for both spinlocks and mutexes. Both types of lock waiters will spin in their own cachelines (in the OSP wait queue in the case of mutex) except one that is at the head of the queue. So there should be minimal cacheline bouncing. One should certainly uses mutexes in sleep-able context or when the critical section is long. Cheers, Longman