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[82.17.115.212]) by smtp.gmail.com with ESMTPSA id j28-20020a05600c1c1c00b003942a244f39sm3382841wms.18.2022.05.04.02.32.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 May 2022 02:32:31 -0700 (PDT) Date: Wed, 4 May 2022 10:32:30 +0100 From: Aaron Tomlin To: Marcelo Tosatti Cc: frederic@kernel.org, cl@linux.com, tglx@linutronix.de, mingo@kernel.org, peterz@infradead.org, pauld@redhat.com, neelx@redhat.com, oleksandr@natalenko.name, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH v3] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too Message-ID: <20220504093230.xmksdyagcpgs7sjt@ava.usersys.com> X-PGP-Key: http://pgp.mit.edu/pks/lookup?search=atomlin%40redhat.com X-PGP-Fingerprint: 7906 84EB FA8A 9638 8D1E 6E9B E2DE 9658 19CC 77D6 References: <20220422193647.3808657-1-atomlin@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 2022-04-28 15:10 -0300, Marcelo Tosatti wrote: > So you are syncing the vmstats on every system call return: Hi Marcelo, Sorry about the delay! No - indeed, that would be too expensive. If I understand correctly, Peter Zijlstra's feedback was in response to a previous suggestion made: "Could we *always* fold the vmstat counters when entering idle mode? ..." I think an exception should be made for the adaptive-tick mode/or a nohz_full CPU case when the scheduling-clock tick is stopped. Also, I feel correctness is key, as previously indicated since a significant divergence can impact memory reclaim code. > Have you measured performance of any system call heavy application > with this change? Unfortunately not. That being said, the aforementioned test and work will only take place under a nohz_full CPU and if the tick is stopped. So this should be somewhat limited, no? > Then the comment on why its so slow: > > "This loop is quite heavy. Maybe reducing the data necessary to be read > to a couple of cachelines would improve it considerably." > > The comment: > > "Is there anything that prevents a nohz full CPU from running an > application with short and frequent idling?" > > Is confusing and can be ignored. Understood. Kind regards, -- Aaron Tomlin