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[140.211.169.12]) by mx.google.com with ESMTPS id u131si2633861pgb.594.2019.01.28.12.10.30 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 28 Jan 2019 12:10:30 -0800 (PST) Received-SPF: pass (google.com: domain of akpm@linux-foundation.org designates 140.211.169.12 as permitted sender) client-ip=140.211.169.12; Authentication-Results: mx.google.com; spf=pass (google.com: domain of akpm@linux-foundation.org designates 140.211.169.12 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org Received: from akpm3.svl.corp.google.com (unknown [104.133.8.65]) by mail.linuxfoundation.org (Postfix) with ESMTPSA id E475623FA; Mon, 28 Jan 2019 20:10:29 +0000 (UTC) Date: Mon, 28 Jan 2019 12:10:28 -0800 From: Andrew Morton To: Rik van Riel Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel-team@fb.com, Johannes Weiner , Chris Mason , Roman Gushchin , Michal Hocko Subject: Re: [PATCH] mm,slab,vmscan: accumulate gradual pressure on small slabs Message-Id: <20190128121028.8ef4c19dd3fd1d60d2e3284c@linux-foundation.org> In-Reply-To: <8ddf2ea674711f373062f4e056dd14fb81c5a2fe.camel@surriel.com> References: <20190128143535.7767c397@imladris.surriel.com> <20190128115424.df3f4647023e9e43e75afe67@linux-foundation.org> <8ddf2ea674711f373062f4e056dd14fb81c5a2fe.camel@surriel.com> X-Mailer: Sylpheed 3.6.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit 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 Mon, 28 Jan 2019 15:03:28 -0500 Rik van Riel wrote: > On Mon, 2019-01-28 at 11:54 -0800, Andrew Morton wrote: > > On Mon, 28 Jan 2019 14:35:35 -0500 Rik van Riel > > wrote: > > > > > /* > > > * Make sure we apply some minimal pressure on default priority > > > - * even on small cgroups. Stale objects are not only consuming > > > memory > > > + * even on small cgroups, by accumulating pressure across > > > multiple > > > + * slab shrinker runs. Stale objects are not only consuming > > > memory > > > * by themselves, but can also hold a reference to a dying > > > cgroup, > > > * preventing it from being reclaimed. A dying cgroup with all > > > * corresponding structures like per-cpu stats and kmem caches > > > * can be really big, so it may lead to a significant waste of > > > memory. > > > */ > > > - delta = max_t(unsigned long long, delta, min(freeable, > > > batch_size)); > > > + if (!delta) { > > > + shrinker->small_scan += freeable; > > > + > > > + delta = shrinker->small_scan >> priority; > > > + shrinker->small_scan -= delta << priority; > > > + > > > + delta *= 4; > > > + do_div(delta, shrinker->seeks); > > > > What prevents shrinker->small_scan from over- or underflowing over > > time? > > We only go into this code path if > delta >> DEF_PRIORITY is zero. > > That is, freeable is smaller than 4096. > I'm still not understanding. If `freeable' always has a value of (say) 1, we'll eventually overflow shrinker->small_scan? Or at least, it's unobvious why this cannot happen.