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Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1fyDoa-00053X-Un; Fri, 07 Sep 2018 10:16:37 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id A4BE8201F9C46; Fri, 7 Sep 2018 12:16:34 +0200 (CEST) Date: Fri, 7 Sep 2018 12:16:34 +0200 From: Peter Zijlstra To: Johannes Weiner Cc: Ingo Molnar , Andrew Morton , Linus Torvalds , Tejun Heo , Suren Baghdasaryan , Daniel Drake , Vinayak Menon , Christopher Lameter , Peter Enderborg , Shakeel Butt , Mike Galbraith , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and IO Message-ID: <20180907101634.GO24106@hirez.programming.kicks-ass.net> References: <20180828172258.3185-1-hannes@cmpxchg.org> <20180828172258.3185-9-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180828172258.3185-9-hannes@cmpxchg.org> User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 28, 2018 at 01:22:57PM -0400, Johannes Weiner wrote: > +enum psi_states { > + PSI_IO_SOME, > + PSI_IO_FULL, > + PSI_MEM_SOME, > + PSI_MEM_FULL, > + PSI_CPU_SOME, > + /* Only per-CPU, to weigh the CPU in the global average: */ > + PSI_NONIDLE, > + NR_PSI_STATES, > +}; > +static u32 get_recent_time(struct psi_group *group, int cpu, > + enum psi_states state) > +{ > + struct psi_group_cpu *groupc = per_cpu_ptr(group->pcpu, cpu); > + unsigned int seq; > + u32 time, delta; > + > + do { > + seq = read_seqcount_begin(&groupc->seq); > + > + time = groupc->times[state]; > + /* > + * 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, state)) > + time += cpu_clock(cpu) - groupc->state_start; > + } while (read_seqcount_retry(&groupc->seq, seq)); > + > + delta = time - groupc->times_prev[state]; > + groupc->times_prev[state] = time; > + > + return delta; > +} > +static bool update_stats(struct psi_group *group) > +{ > + u64 deltas[NR_PSI_STATES - 1] = { 0, }; > + unsigned long missed_periods = 0; > + unsigned long nonidle_total = 0; > + u64 now, expires, period; > + int cpu; > + int s; > + > + mutex_lock(&group->stat_lock); > + > + /* > + * Collect the per-cpu time buckets and average them into a > + * single time sample that is normalized to wallclock time. > + * > + * For averaging, each CPU is weighted by its non-idle time in > + * the sampling period. This eliminates artifacts from uneven > + * loading, or even entirely idle CPUs. > + */ > + for_each_possible_cpu(cpu) { > + u32 nonidle; > + > + nonidle = get_recent_time(group, cpu, PSI_NONIDLE); > + nonidle = nsecs_to_jiffies(nonidle); > + nonidle_total += nonidle; > + > + for (s = 0; s < PSI_NONIDLE; s++) { > + u32 delta; > + > + delta = get_recent_time(group, cpu, s); > + deltas[s] += (u64)delta * nonidle; > + } > + } This does the whole seqcount thing 6x, which is a bit of a waste. struct snapshot { u32 times[NR_PSI_STATES]; }; static inline struct snapshot get_times_snapshot(struct psi_group *pg, int cpu) { struct pci_group_cpu *pgc = per_cpu_ptr(pg->pcpu, cpu); struct snapshot s; unsigned int seq; u32 delta; int i; do { seq = read_seqcount_begin(&pgc->seq); delta = cpu_clock(cpu) - pgc->state_start; for (i = 0; i < NR_PSI_STATES; i++) { s.times[i] = gpc->times[i]; if (test_state(pgc->tasks, i)) s.times[i] += delta; } } while (read_seqcount_retry(&pgc->seq, seq); return s; } for_each_possible_cpu(cpu) { struct snapshot s = get_times_snapshot(pg, cpu); nonidle = nsecs_to_jiffies(s.times[PSI_NONIDLE]); nonidle_total += nonidle; for (i = 0; i < PSI_NONIDLE; i++) deltas[s] += (u64)s.times[i] * nonidle; /* ... */ } It's a bit cumbersome, but that's because of C.