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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id d8-20020a81d808000000b00545a08184dcsm3092357ywj.108.2023.03.29.12.22.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Mar 2023 12:22:35 -0700 (PDT) Date: Wed, 29 Mar 2023 12:22:24 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.attlocal.net To: Tejun Heo cc: Yosry Ahmed , Shakeel Butt , Josef Bacik , Jens Axboe , Zefan Li , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Andrew Morton , Vasily Averin , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, bpf@vger.kernel.org Subject: Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the percpu lock In-Reply-To: Message-ID: References: <20230323040037.2389095-1-yosryahmed@google.com> <20230323040037.2389095-2-yosryahmed@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: DBCE040009 X-Stat-Signature: w5r1bdzm8ft43xgyskgrjtx5874skaek X-HE-Tag: 1680117757-354040 X-HE-Meta: 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 UyeIMe12 ab/S/5+VWkr+e5qIjeGsiq+BDv9Metr020iuv1c4BLS07EuBcEkDjxwqgx1xlItKY4VvBcTktcrjF+mKD8CcL2ZP3kKFkxT52n+B0Vpwsm5wkZaqJjoBvjhrgDTgnOOqZY2M8YW3NYuMBgyFnu218BNcUpZN/UG1ieOg59cZ4eDS30ibA8xCNsf112Be1AJU0hPl+S4XknD2P+vNsQ8Fpy2idkhXlF9r7uq3ebB2mWIY4JJaPsI/K8Bd7yuIMEZot4UUIYQM01KL+ly+YVwbAPcx4rMDESZ1AA0duiclQGlP8gEHLp4YrTMj3bKx3xP5Q2cHPL10SfMTeRbR8G6B9Y6K04lhrO02hUKtEwT3/5h95IW3cQLUwL+Ld9BPWSXvDV2HG 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: Hi Tejun, On Wed, 29 Mar 2023, Tejun Heo wrote: > On Mon, Mar 27, 2023 at 04:23:13PM -0700, Yosry Ahmed wrote: > > Tejun, if having the lock be non-irq is a non-starter for you, I can > > This is an actual hazard. We see in prod these unprotected locks causing > very big spikes in tail latencies and they can be tricky to root cause too > and given the way rstat lock is used it's highly likely to be involved in > those scenarios with the proposed change, so it's gonna be a nack from my > end. Butting in here, I'm fascinated. This is certainly not my area, I know nothing about rstat, but this is the first time I ever heard someone arguing for more disabling of interrupts rather than less. An interrupt coming in while holding a contended resource can certainly add to latencies, that I accept of course. But until now, I thought it was agreed best practice to disable irqs only regretfully, when strictly necessary. If that has changed, I for one want to know about it. How should we now judge which spinlocks should disable interrupts and which should not? Page table locks are currently my main interest - should those be changed? Thanks, Hugh