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Mon, 22 Aug 2022 10:18:34 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id gluMKvpXA2POCgAAMHmgww (envelope-from ); Mon, 22 Aug 2022 10:18:34 +0000 Date: Mon, 22 Aug 2022 12:18:34 +0200 From: Michal Hocko To: Shakeel Butt Cc: Johannes Weiner , Roman Gushchin , Muchun Song , Michal =?iso-8859-1?Q?Koutn=FD?= , Eric Dumazet , Soheil Hassas Yeganeh , Feng Tang , Oliver Sang , Andrew Morton , lkp@lists.01.org, cgroups@vger.kernel.org, linux-mm@kvack.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] mm: page_counter: remove unneeded atomic ops for low/min Message-ID: References: <20220822001737.4120417-1-shakeelb@google.com> <20220822001737.4120417-2-shakeelb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: ARC-Authentication-Results: i=1; imf31.hostedemail.com; dkim=pass header.d=suse.com header.s=susede1 header.b=l2HPQA8o; spf=pass (imf31.hostedemail.com: domain of mhocko@suse.com designates 195.135.220.29 as permitted sender) smtp.mailfrom=mhocko@suse.com; 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spf=pass (imf31.hostedemail.com: domain of mhocko@suse.com designates 195.135.220.29 as permitted sender) smtp.mailfrom=mhocko@suse.com; dmarc=pass (policy=quarantine) header.from=suse.com X-Rspamd-Server: rspam02 X-Stat-Signature: t83sy5dcgoxzajubtnk18y5njeztmmax X-Rspamd-Queue-Id: 09A272004A X-HE-Tag: 1661163515-975177 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 22-08-22 11:55:33, Michal Hocko wrote: > On Mon 22-08-22 00:17:35, Shakeel Butt wrote: [...] > > diff --git a/mm/page_counter.c b/mm/page_counter.c > > index eb156ff5d603..47711aa28161 100644 > > --- a/mm/page_counter.c > > +++ b/mm/page_counter.c > > @@ -17,24 +17,23 @@ static void propagate_protected_usage(struct page_counter *c, > > unsigned long usage) > > { > > unsigned long protected, old_protected; > > - unsigned long low, min; > > long delta; > > > > if (!c->parent) > > return; > > > > - min = READ_ONCE(c->min); > > - if (min || atomic_long_read(&c->min_usage)) { > > - protected = min(usage, min); > > + protected = min(usage, READ_ONCE(c->min)); > > + old_protected = atomic_long_read(&c->min_usage); > > + if (protected != old_protected) { > > I have to cache that code back into brain. It is really subtle thing and > it is not really obvious why this is still correct. I will think about > that some more but the changelog could help with that a lot. OK, so the this patch will be most useful when the min > 0 && min < usage because then the protection doesn't really change since the last call. In other words when the usage grows above the protection and your workload benefits from this change because that happens a lot as only a part of the workload is protected. Correct? Unless I have missed anything this shouldn't break the correctness but I still have to think about the proportional distribution of the protection because that adds to the complexity here. -- Michal Hocko SUSE Labs