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[2607:7c80:54:e::133]) by mx.google.com with ESMTPS id v12si910721pgr.454.2019.04.24.05.15.53 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 24 Apr 2019 05:15:53 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of willy@infradead.org designates 2607:7c80:54:e::133 as permitted sender) client-ip=2607:7c80:54:e::133; Authentication-Results: mx.google.com; dkim=pass header.i=@infradead.org header.s=bombadil.20170209 header.b=J8NPq1X7; spf=pass (google.com: best guess record for domain of willy@infradead.org designates 2607:7c80:54:e::133 as permitted sender) smtp.mailfrom=willy@infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=vj3d8vRWRgl1TUYLwBnfGWFxKdoE7W/fi9Cn4NFB8No=; b=J8NPq1X7WmSDNtrLizSrfa18N glrdrjtxQ84BeviuPHlUcWxnpwS5ifwUV8UrwJOwCe8jwYAahVStfF5QCj7r1SeCgfvG0puP4kR8e kMNY1nHHZpt66U40Yfz/CtUV1mG7lAzp8cAAPmd/oxsAqgJ5RUZarP73PVhXo/AiXPcSiA1rBnYvi aeXxqDYZqKWFsbTmnL1zJrVMuu7Qk912EloZedAFDKqCM0ab9VA0vCAkGjERb7JdZ0YlJzezzBXw5 P7+ByEpqDtNHMxgHAjDDQ0I2pFsyFmvxU1RXViI1sariHkEgsDRhy8HCFkgZ66M+Bxthf6jFllLcz nsR030Thw==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1hJGoa-0001PG-Dz; Wed, 24 Apr 2019 12:15:52 +0000 Date: Wed, 24 Apr 2019 05:15:52 -0700 From: Matthew Wilcox To: Sebastian Andrzej Siewior Cc: linux-mm@kvack.org, tglx@linutronix.de, frederic@kernel.org, Christoph Lameter , anna-maria@linutronix.de Subject: Re: [PATCH 0/4 v2] mm/swap: Add locking for pagevec Message-ID: <20190424121552.GD19031@bombadil.infradead.org> References: <20190424111208.24459-1-bigeasy@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190424111208.24459-1-bigeasy@linutronix.de> User-Agent: Mutt/1.9.2 (2017-12-15) 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 Wed, Apr 24, 2019 at 01:12:04PM +0200, Sebastian Andrzej Siewior wrote: > The swap code synchronizes its access to the (four) pagevec struct > (which is allocated per-CPU) by disabling preemption. This works and the > one struct needs to be accessed from interrupt context is protected by > disabling interrupts. This was manually audited and there is no lockdep > coverage for this. > There is one case where the per-CPU of a remote CPU needs to be accessed > and this is solved by started a worker on the remote CPU and waiting for > it to finish. > > In v1 [0] it was attempted to add per-CPU spinlocks for the access to > struct. This would add lockdep coverage and access from a remote CPU so > the worker wouldn't be required. >From my point of view, what is missing from this description is why we want to be able to access these structs from a remote CPU. It's explained a little better in the 4/4 changelog, but I don't see any numbers that suggest what kinds of gains we might see (eg "reduces power consumption by x% on a particular setup", or even "average length of time in idle extended from x ms to y ms").