From mboxrd@z Thu Jan 1 00:00:00 1970 From: Roman Gushchin Subject: Re: [PATCH] memcg: expose root cgroup's memory.stat Date: Fri, 15 May 2020 08:00:26 -0700 Message-ID: <20200515150026.GA94522@carbon.DHCP.thefacebook.com> References: <20200508170630.94406-1-shakeelb@google.com> <20200508214405.GA226164@cmpxchg.org> <20200515082955.GJ29153@dhcp22.suse.cz> <20200515132421.GC591266@cmpxchg.org> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=fb.com; h=date : from : to : cc : subject : message-id : references : content-type : in-reply-to : mime-version; s=facebook; bh=FXFoHMcftAIBfyNkY53nBPS4LdgInjr/xrQffhf24K0=; b=peCNMd+/qci3NvmEQOI2kBqhxxywoWn0pZjQ5W7s89K006cxKDkjkgPqrRjEzSHBLGtO ZS1cxJqn752AMFs1DFVIKfilNs/X/Q/jbLblRu6hYf/DQVbaMksuSclCHHUIuJaGDIdh 2fZxc22B+OcdVqEMIcTV4HEwpQH1OBMz2EA= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=fb.onmicrosoft.com; s=selector2-fb-onmicrosoft-com; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck; bh=FXFoHMcftAIBfyNkY53nBPS4LdgInjr/xrQffhf24K0=; b=EdiXJgHSKPV/6gtXdM7sOScafdrVk4Qb7u3cuubR6eG/wn9F2YzAXc8NpgY3pSbeqeFwCdC4Xub+BdTYSfkJwV+8zd9XX/feTtJZ+CLuqALzv8e3ZCpDE5UDITwDgxzV+HppSOXRuZAADIdyQaKxlOpbxChoFmQmBmhYmXZaHYs= Content-Disposition: inline In-Reply-To: Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Shakeel Butt Cc: Johannes Weiner , Michal Hocko , Mel Gorman , Andrew Morton , Yafang Shao , Linux MM , Cgroups , LKML On Fri, May 15, 2020 at 06:44:44AM -0700, Shakeel Butt wrote: > On Fri, May 15, 2020 at 6:24 AM Johannes Weiner wrote: > > > > On Fri, May 15, 2020 at 10:29:55AM +0200, Michal Hocko wrote: > > > 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. > > > > Yep, we skip the page_counter for root, but keep in mind that cgroup1 > > *does* have a root-level memory.stat, so (for the most part) we've > > been keeping consumer stats for the root level the whole time. > > > > > counters because they are not really all the same. E.g. > > > - mem_cgroup_charge_statistics accounts for each memcg > > > > Yep, that's heritage from cgroup1. > > > > > - 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 > > > > You're right. It should only bypass the page_counter, but still set > > page->mem_cgroup = root_mem_cgroup, just like user pages. What about kernel threads? We consider them belonging to the root memory cgroup. Should their memory consumption being considered in root-level stats? I'm not sure we really want it, but I guess we need to document how kernel threads are handled. > > > > This counter also doesn't get exported on cgroup1, so it would indeed > > be a new bug. It needs to be fixed before this patch here. > > > > > - memcg_charge_slab (NR_SLAB*) skips over root memcg as well > > > > Same thing with these two. > > Yes, we skip page_counter for root but not the stats. I will go over > all the stats and make sure we are not skipping the stats for root.