From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f200.google.com (mail-pf1-f200.google.com [209.85.210.200]) by kanga.kvack.org (Postfix) with ESMTP id 7FC226B2397 for ; Wed, 22 Aug 2018 05:16:49 -0400 (EDT) Received: by mail-pf1-f200.google.com with SMTP id n17-v6so833542pff.17 for ; Wed, 22 Aug 2018 02:16:49 -0700 (PDT) Received: from bombadil.infradead.org (bombadil.infradead.org. [2607:7c80:54:e::133]) by mx.google.com with ESMTPS id 15-v6si1277529pgu.205.2018.08.22.02.16.48 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 22 Aug 2018 02:16:48 -0700 (PDT) Date: Wed, 22 Aug 2018 11:16:40 +0200 From: Peter Zijlstra Subject: Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and IO Message-ID: <20180822091640.GV24124@hirez.programming.kicks-ass.net> References: <20180801151958.32590-1-hannes@cmpxchg.org> <20180801151958.32590-9-hannes@cmpxchg.org> <20180803165641.GA2476@hirez.programming.kicks-ass.net> <20180821194413.GA24538@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180821194413.GA24538@cmpxchg.org> Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: Ingo Molnar , Andrew Morton , Linus Torvalds , Tejun Heo , Suren Baghdasaryan , Daniel Drake , Vinayak Menon , Christopher Lameter , Mike Galbraith , Shakeel Butt , Peter Enderborg , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com On Tue, Aug 21, 2018 at 03:44:13PM -0400, Johannes Weiner wrote: > > > + for (s = PSI_NONIDLE; s >= 0; s--) { > > > + u32 time, delta; > > > + > > > + time = READ_ONCE(groupc->times[s]); > > > + /* > > > + * In addition to already concluded states, we > > > + * also incorporate currently active states on > > > + * the CPU, since states may last for many > > > + * sampling periods. > > > + * > > > + * This way we keep our delta sampling buckets > > > + * small (u32) and our reported pressure close > > > + * to what's actually happening. > > > + */ > > > + if (test_state(groupc->tasks, cpu, s)) { > > > + /* > > > + * We can race with a state change and > > > + * need to make sure the state_start > > > + * update is ordered against the > > > + * updates to the live state and the > > > + * time buckets (groupc->times). > > > + * > > > + * 1. If we observe task state that > > > + * needs to be recorded, make sure we > > > + * see state_start from when that > > > + * state went into effect or we'll > > > + * count time from the previous state. > > > + * > > > + * 2. If the time delta has already > > > + * been added to the bucket, make sure > > > + * we don't see it in state_start or > > > + * we'll count it twice. > > > + * > > > + * If the time delta is out of > > > + * state_start but not in the time > > > + * bucket yet, we'll miss it entirely > > > + * and handle it in the next period. > > > + */ > > > + smp_rmb(); > > > + time += cpu_clock(cpu) - groupc->state_start; > > > + } > > > > The alternative is adding an update to scheduler_tick(), that would > > ensure you're never more than nr_cpu_ids * TICK_NSEC behind. > > I wasn't able to convert *all* states to tick updates like this. > > The reason is that, while testing rq->curr for PF_MEMSTALL is cheap, > other tasks associated with the rq could be from any cgroup in the > system. That means we'd have to do for_each_cgroup() on every tick to > keep the groupc->times that closely uptodate, and that wouldn't scale. > We tend to have hundreds of them, some setups have thousands. > > Since we don't need to be *that* current, I left the on-demand update > inside the aggregator for now. It's a bit trickier, but much cheaper. ARGH indeed; I was thinking we only need to update current. But because we're tracking blocked state that doesn't work. Sorry for that :/