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[37.188.249.36]) by smtp.gmail.com with ESMTPSA id h137sm54689649wme.0.2020.05.15.01.29.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 15 May 2020 01:29:56 -0700 (PDT) Date: Fri, 15 May 2020 10:29:55 +0200 From: Michal Hocko To: Shakeel Butt Cc: Johannes Weiner , Mel Gorman , Roman Gushchin , Andrew Morton , Yafang Shao , Linux MM , Cgroups , LKML Subject: Re: [PATCH] memcg: expose root cgroup's memory.stat Message-ID: <20200515082955.GJ29153@dhcp22.suse.cz> References: <20200508170630.94406-1-shakeelb@google.com> <20200508214405.GA226164@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat 09-05-20 07:06:38, Shakeel Butt wrote: > On Fri, May 8, 2020 at 2:44 PM Johannes Weiner wrote: > > > > On Fri, May 08, 2020 at 10:06:30AM -0700, Shakeel Butt wrote: > > > One way to measure the efficiency of memory reclaim is to look at the > > > ratio (pgscan+pfrefill)/pgsteal. However at the moment these stats are > > > not updated consistently at the system level and the ratio of these are > > > not very meaningful. The pgsteal and pgscan are updated for only global > > > reclaim while pgrefill gets updated for global as well as cgroup > > > reclaim. > > > > > > Please note that this difference is only for system level vmstats. The > > > cgroup stats returned by memory.stat are actually consistent. The > > > cgroup's pgsteal contains number of reclaimed pages for global as well > > > as cgroup reclaim. So, one way to get the system level stats is to get > > > these stats from root's memory.stat, so, expose memory.stat for the root > > > cgroup. > > > > > > from Johannes Weiner: > > > There are subtle differences between /proc/vmstat and > > > memory.stat, and cgroup-aware code that wants to watch the full > > > hierarchy currently has to know about these intricacies and > > > translate semantics back and forth. Can we have those subtle differences documented please? > > > > > > Generally having the fully recursive memory.stat at the root > > > level could help a broader range of usecases. > > > > The changelog begs the question why we don't just "fix" the > > system-level stats. It may be useful to include the conclusions from > > that discussion, and why there is value in keeping the stats this way. > > > > Right. Andrew, can you please add the following para to the changelog? > > Why not fix the stats by including both the global and cgroup reclaim > activity instead of exposing root cgroup's memory.stat? The reason is > the benefit of having metrics exposing the activity that happens > purely due to machine capacity rather than localized activity that > happens due to the limits throughout the cgroup tree. Additionally > there are userspace tools like sysstat(sar) which reads these stats to > inform about the system level reclaim activity. So, we should not > break such use-cases. > > > > Signed-off-by: Shakeel Butt > > > Suggested-by: Johannes Weiner > > > > Acked-by: Johannes Weiner > > Thanks a lot. I was quite surprised that the patch is so simple TBH. For some reason I've still had memories that we do not account for root memcg (likely because mem_cgroup_is_root(memcg) bail out in the try_charge. But stats are slightly different here. I have started looking at different stat counters because they are not really all the same. E.g. - mem_cgroup_charge_statistics accounts for each memcg - memcg_charge_kernel_stack relies on pages being associated with a memcg and that in turn relies on __memcg_kmem_charge_page which bails out on root memcg - memcg_charge_slab (NR_SLAB*) skips over root memcg as well - __mod_lruvec_page_state relies on page->mem_cgroup as well but this one is ok for paths which go through commit_charge path. That being said we should really double check which stats are accounted properly. At least MEMCG_KERNEL_STACK_KB won't unless I am misreading the code. I do not mind displaying the root's memcg stats but a) a closer look had to be done for each counter and b) a clarification of differences from the global vmstat counters would be really handy. -- Michal Hocko SUSE Labs