From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH] mm/memcg: Disable task obj_stock for PREEMPT_RT Date: Wed, 4 Aug 2021 10:33:39 +0200 Message-ID: References: <20210803175519.22298-1-longman@redhat.com> <87h7g62jxm.ffs@tglx> <93dfe0d4-c687-93f8-da75-c5d3c9bd0ac7@suse.cz> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1628066021; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=8fZuu5IPh8/NDuL9TS9G3aL/MepY8Xyguc7Af8DztIg=; b=ouxwYpdxC1RedfFp8dj0o3PNLQCFDo9OUoIE7ySDg/ScOugzvVH0ZKfgL4NMwE/lCvtVat LPIEqispUA9aIH2ESfpT3QYs1eQEEC/YhjAYan/dVOC9c9eEiPuVyfJNyvaOoSuS81rgzT J5Y4CLyDH2KnTqWZVkfZB0LHCTFyGQM= Content-Disposition: inline In-Reply-To: <93dfe0d4-c687-93f8-da75-c5d3c9bd0ac7@suse.cz> List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Vlastimil Babka Cc: Thomas Gleixner , Waiman Long , Johannes Weiner , Vladimir Davydov , Andrew Morton , Roman Gushchin , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, Shakeel Butt , Muchun Song , Luis Goncalves , Sebastian Andrzej Siewior , Daniel Bristot de Oliveira , Linus Torvalds On Wed 04-08-21 09:39:23, Vlastimil Babka wrote: > On 8/4/21 1:21 AM, Thomas Gleixner wrote: > > /* > > * The only protection from memory hotplug vs. drain_stock races is > > * that we always operate on local CPU stock here with IRQ disabled > > */ > > - local_irq_save(flags); > > + local_lock_irqsave(memcg_stock_lock, flags); > > ... > > if (use_task_obj_stock()) > > drain_obj_stock(&stock->task_obj); > > > > which is incomprehensible garbage. > > > > The comment above the existing local_irq_save() is garbage w/o any local > > lock conversion already today (and even before the commit which > > introduced stock::task_obj) simply because that comment does not explain > > the why. > > Michal, this seems to be your comment from commit 72f0184c8a00 ("mm, memcg: > remove hotplug locking from try_charge"). Was "memory hotplug" a mistake, > because the rest of the commit is about cpu hotplug, and I don't really see a > memory hotplug connection there? This part of the changelog tried to explain that part IIRC " We can get rid of {get,put}_online_cpus, fortunately. We do not have to be worried about races with memory hotplug because drain_local_stock, which is called from both the WQ draining and the memory hotplug contexts, is always operating on the local cpu stock with IRQs disabled. " Now I have to admit I do not remember all the details and from a quick look the memory hotplug doesn't seem to be draining memcg pcp stock. Maybe this has been removed since then. The only stock draining outside of the memcg code seems to be memcg_hotplug_cpu_dead callback. That would indicate that I really meant the cpu hotplug here indeed. -- Michal Hocko SUSE Labs