From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id DBBFFC433EF for ; Mon, 21 Feb 2022 17:17:43 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id F09AF8D0002; Mon, 21 Feb 2022 12:17:42 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id EB7738D0001; Mon, 21 Feb 2022 12:17:42 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D58718D0002; Mon, 21 Feb 2022 12:17:42 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0185.hostedemail.com [216.40.44.185]) by kanga.kvack.org (Postfix) with ESMTP id C7D548D0001 for ; Mon, 21 Feb 2022 12:17:42 -0500 (EST) Received: from smtpin08.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 7A0A8181B8AF5 for ; Mon, 21 Feb 2022 17:17:42 +0000 (UTC) X-FDA: 79167443964.08.C9F56B6 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by imf11.hostedemail.com (Postfix) with ESMTP id EDA474000B for ; Mon, 21 Feb 2022 17:17:41 +0000 (UTC) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id BFE6121121; Mon, 21 Feb 2022 17:17:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1645463860; 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=+7KYGytx3sPksKcMmFvof7rFmXeF2SkMG7FYcunC5P8=; b=dIABDJkQzMckrp5l6PQDAReBA7SrzSTO3wEN2+K6vZSdcjenF7wfKKpetyIehJcs5C3fs2 wcXKQm/bf+xWIDtdZw/2AFeodvzTbiCQXroiQ6+UPgfwQvNYL45yJMD6d6OkuXu0Eqg2SH GBpZ8mnn4HJFl6AZBmzjD2tdwnTLNC0= Received: from suse.cz (unknown [10.100.201.86]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 860F8A3B8A; Mon, 21 Feb 2022 17:17:40 +0000 (UTC) Date: Mon, 21 Feb 2022 18:17:39 +0100 From: Michal Hocko To: Sebastian Andrzej Siewior Cc: cgroups@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Johannes Weiner , Michal =?iso-8859-1?Q?Koutn=FD?= , Peter Zijlstra , Thomas Gleixner , Vladimir Davydov , Waiman Long , kernel test robot Subject: Re: [PATCH v3 5/5] mm/memcg: Protect memcg_stock with a local_lock_t Message-ID: References: <20220217094802.3644569-1-bigeasy@linutronix.de> <20220217094802.3644569-6-bigeasy@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: Authentication-Results: imf11.hostedemail.com; dkim=pass header.d=suse.com header.s=susede1 header.b=dIABDJkQ; spf=pass (imf11.hostedemail.com: domain of mhocko@suse.com designates 195.135.220.28 as permitted sender) smtp.mailfrom=mhocko@suse.com; dmarc=pass (policy=quarantine) header.from=suse.com X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: EDA474000B X-Stat-Signature: s6ewftm358pga4eusxdoykmp6i1oqiue X-HE-Tag: 1645463861-790298 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000139, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon 21-02-22 17:44:13, Sebastian Andrzej Siewior wrote: > On 2022-02-21 17:24:41 [+0100], Michal Hocko wrote: > > > > > @@ -2282,14 +2288,9 @@ static void drain_all_stock(struct mem_cgroup *root_memcg) > > > > > rcu_read_unlock(); > > > > > > > > > > if (flush && > > > > > - !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) { > > > > > - if (cpu == curcpu) > > > > > - drain_local_stock(&stock->work); > > > > > - else > > > > > - schedule_work_on(cpu, &stock->work); > > > > > - } > > > > > + !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) > > > > > + schedule_work_on(cpu, &stock->work); > > > > > > > > Maybe I am missing but on !PREEMPT kernels there is nothing really > > > > guaranteeing that the worker runs so there should be cond_resched after > > > > the mutex is unlocked. I do not think we want to rely on callers to be > > > > aware of this subtlety. > > > > > > There is no guarantee on PREEMPT kernels, too. The worker will be made > > > running and will be put on the CPU when the scheduler sees it fit and > > > there could be other worker which take precedence (queued earlier). > > > But I was not aware that the worker _needs_ to run before we return. > > > > A lack of draining will not be a correctness problem (sorry I should > > have made that clear). It is more about subtlety than anything. E.g. the > > charging path could be forced to memory reclaim because of the cached > > charges which are still waiting for their draining. Not really something > > to lose sleep over from the runtime perspective. I was just wondering > > that this makes things more complex than necessary. > > So it is no strictly wrong but it would be better if we could do > drain_local_stock() on the local CPU. > > > > We > > > might get migrated after put_cpu() so I wasn't aware that this is > > > important. Should we attempt best effort and wait for the worker on the > > > current CPU? > > > > > > > > An alternative would be to split out __drain_local_stock which doesn't > > > > do local_lock. > > > > > > but isn't the section in drain_local_stock() unprotected then? > > > > local_lock instead of {get,put}_cpu would handle that right? > > It took a while, but it clicked :) > If we acquire the lock_lock_t, that we would otherwise acquire in > drain_local_stock(), before the for_each_cpu loop (as you say > get,pu_cpu) then we would indeed need __drain_local_stock() and things > would work. But it looks like an abuse of the lock to avoid CPU > migration since there is no need to have it acquired at this point. Also > the whole section would run with disabled interrupts and there is no > need for it. > > What about if replace get_cpu() with migrate_disable()? Yeah, that would be a better option. I am just not used to think in RT so migrate_disable didn't really come to my mind. -- Michal Hocko SUSE Labs