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From: Kent Overstreet To: Mel Gorman Cc: Suren Baghdasaryan , akpm@linux-foundation.org, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, roman.gushchin@linux.dev, dave@stgolabs.net, willy@infradead.org, liam.howlett@oracle.com, void@manifault.com, peterz@infradead.org, juri.lelli@redhat.com, ldufour@linux.ibm.com, peterx@redhat.com, david@redhat.com, axboe@kernel.dk, mcgrof@kernel.org, masahiroy@kernel.org, nathan@kernel.org, changbin.du@intel.com, ytcoode@gmail.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, bristot@redhat.com, vschneid@redhat.com, cl@linux.com, penberg@kernel.org, iamjoonsoo.kim@lge.com, 42.hyeyoo@gmail.com, glider@google.com, elver@google.com, dvyukov@google.com, shakeelb@google.com, songmuchun@bytedance.com, arnd@arndb.de, jbaron@akamai.com, rientjes@google.com, minchan@google.com, kaleshsingh@google.com, kernel-team@android.com, linux-mm@kvack.org, iommu@lists.linux.dev, kasan-dev@googlegroups.com, io-uring@vger.kernel.org, linux-arch@vger.kernel.org, xen-devel@lists.xenproject.org, linux-bcache@vger.kernel.org, linux-modules@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 03/30] Lazy percpu counters Message-ID: <20220831162030.hzgzhxu3qn6g3k5r@moria.home.lan> References: <20220830214919.53220-1-surenb@google.com> <20220830214919.53220-4-surenb@google.com> <20220831100249.f2o27ri7ho4ma3pe@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220831100249.f2o27ri7ho4ma3pe@suse.de> X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1661962838; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=iGyIm0136Cqvu8OauWzfpc3vydsg6hlY8VzCbW2wXJg=; b=3AA3Ibu1jhCfwXbBpV28p0IJLzMs0iLdb47y4k/ANGyBDlfY6C9I1g0/q6q2jxeS46GlIW 5cChD3Ig/NDyfG9fU8Y16wlSERKzs7DJNG1jt51dJw6FaIavFbg2CVPPYH5cQHtdR+ZZfK 4UVenNTJOrlJXpxRCDXYoa/YIAlM3/Y= ARC-Authentication-Results: i=1; imf01.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=KVP2c5IA; spf=pass (imf01.hostedemail.com: domain of kent.overstreet@linux.dev designates 91.121.223.63 as permitted sender) smtp.mailfrom=kent.overstreet@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1661962838; a=rsa-sha256; cv=none; b=keIq5Yt8bSqZAZcc2nZeOlPhWkk3j0wYTqph+8T8O/kdtG5EVR4ibNJf+e+p90GdZrPFyE udbWc/vyHF9Jd6L8NAAY6dsoN/katYExtAqlhsP9f5Ab5JJTl7rE3VlnYDEZ46QUbv1vX8 dH4v+qZ3TFpqfsMRQtVOMBDlRqA6CSk= X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 9A3884004F X-Rspam-User: Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=KVP2c5IA; spf=pass (imf01.hostedemail.com: domain of kent.overstreet@linux.dev designates 91.121.223.63 as permitted sender) smtp.mailfrom=kent.overstreet@linux.dev; dmarc=pass (policy=none) header.from=linux.dev X-Stat-Signature: snriiwec8rqhht6ipdn7u4mwmeqtq1a9 X-HE-Tag: 1661962838-328093 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, Aug 31, 2022 at 11:02:49AM +0100, Mel Gorman wrote: > On Tue, Aug 30, 2022 at 02:48:52PM -0700, Suren Baghdasaryan wrote: > > From: Kent Overstreet > > > > This patch adds lib/lazy-percpu-counter.c, which implements counters > > that start out as atomics, but lazily switch to percpu mode if the > > update rate crosses some threshold (arbitrarily set at 256 per second). > > > > Signed-off-by: Kent Overstreet > > Why not use percpu_counter? It has a per-cpu counter that is synchronised > when a batch threshold (default 32) is exceeded and can explicitly sync > the counters when required assuming the synchronised count is only needed > when reading debugfs. It doesn't switch from atomic mode to percpu mode when the update rate crosses a threshold like lazy percpu counters does, it allocates all the percpu counters up front - that makes it a non starter here. Also, from my reading of the code... wtf is it even doing, and why would I use it at all? This looks like old grotty code from ext3, it's not even using this_cpu_add() - it does preempt_enable()/disable() just for adding to a local percpu counter! Noooooope.