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 144206B026B for ; Fri, 3 Aug 2018 13:21:47 -0400 (EDT) Received: by mail-pf1-f200.google.com with SMTP id p5-v6so4040465pfh.11 for ; Fri, 03 Aug 2018 10:21:47 -0700 (PDT) Received: from bombadil.infradead.org (bombadil.infradead.org. [2607:7c80:54:e::133]) by mx.google.com with ESMTPS id bg9-v6si4386534plb.243.2018.08.03.10.21.45 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Fri, 03 Aug 2018 10:21:46 -0700 (PDT) Date: Fri, 3 Aug 2018 19:21:39 +0200 From: Peter Zijlstra Subject: Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and IO Message-ID: <20180803172139.GE2494@hirez.programming.kicks-ass.net> References: <20180801151958.32590-1-hannes@cmpxchg.org> <20180801151958.32590-9-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180801151958.32590-9-hannes@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 Wed, Aug 01, 2018 at 11:19:57AM -0400, Johannes Weiner wrote: > + 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; > + } As is, groupc->state_start needs a READ_ONCE() above and a WRITE_ONCE() below. But like stated earlier, doing an update in scheduler_tick() is probably easier. > +static void psi_group_change(struct psi_group *group, int cpu, u64 now, > + unsigned int clear, unsigned int set) > +{ > + struct psi_group_cpu *groupc; > + unsigned int t, m; > + u32 delta; > + > + groupc = per_cpu_ptr(group->pcpu, cpu); > + > + /* > + * First we assess the aggregate resource states these CPU's > + * tasks have been in since the last change, and account any > + * SOME and FULL time that may have resulted in. > + * > + * Then we update the task counts according to the state > + * change requested through the @clear and @set bits. > + */ > + > + delta = now - groupc->state_start; > + groupc->state_start = now; > + > + /* > + * Update state_start before recording time in the sampling > + * buckets and changing task counts, to prevent a racing > + * aggregation from counting the delta twice or attributing it > + * to an old state. > + */ > + smp_wmb(); > + > + if (test_state(groupc->tasks, cpu, PSI_IO_SOME)) { > + groupc->times[PSI_IO_SOME] += delta; > + if (test_state(groupc->tasks, cpu, PSI_IO_FULL)) > + groupc->times[PSI_IO_FULL] += delta; > + } > + if (test_state(groupc->tasks, cpu, PSI_MEM_SOME)) { > + groupc->times[PSI_MEM_SOME] += delta; > + if (test_state(groupc->tasks, cpu, PSI_MEM_FULL)) > + groupc->times[PSI_MEM_FULL] += delta; > + } Might we worth checking the compiler does the right thing here and optimizes this branch fest into something sensible. > + if (test_state(groupc->tasks, cpu, PSI_CPU_SOME)) > + groupc->times[PSI_CPU_SOME] += delta; > + if (test_state(groupc->tasks, cpu, PSI_NONIDLE)) > + groupc->times[PSI_NONIDLE] += delta;