From: Peter Zijlstra <peterz@infradead.org>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Ingo Molnar <mingo@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Tejun Heo <tj@kernel.org>, Suren Baghdasaryan <surenb@google.com>,
Daniel Drake <drake@endlessm.com>,
Vinayak Menon <vinmenon@codeaurora.org>,
Christopher Lameter <cl@linux.com>,
Mike Galbraith <efault@gmx.de>,
Shakeel Butt <shakeelb@google.com>,
Peter Enderborg <peter.enderborg@sony.com>,
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
Date: Fri, 3 Aug 2018 18:56:41 +0200 [thread overview]
Message-ID: <20180803165641.GA2476@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180801151958.32590-9-hannes@cmpxchg.org>
On Wed, Aug 01, 2018 at 11:19:57AM -0400, Johannes Weiner wrote:
> +static bool test_state(unsigned int *tasks, int cpu, enum psi_states state)
> +{
> + switch (state) {
> + case PSI_IO_SOME:
> + return tasks[NR_IOWAIT];
> + case PSI_IO_FULL:
> + return tasks[NR_IOWAIT] && !tasks[NR_RUNNING];
> + case PSI_MEM_SOME:
> + return tasks[NR_MEMSTALL];
> + case PSI_MEM_FULL:
> + /*
> + * Since we care about lost potential, things are
> + * fully blocked on memory when there are no other
> + * working tasks, but also when the CPU is actively
> + * being used by a reclaimer and nothing productive
> + * could run even if it were runnable.
> + */
> + return tasks[NR_MEMSTALL] &&
> + (!tasks[NR_RUNNING] ||
> + cpu_curr(cpu)->flags & PF_MEMSTALL);
I don't think you can do this, there is nothing that guarantees
cpu_curr() still exists.
> + case PSI_CPU_SOME:
> + return tasks[NR_RUNNING] > 1;
> + case PSI_NONIDLE:
> + return tasks[NR_IOWAIT] || tasks[NR_MEMSTALL] ||
> + tasks[NR_RUNNING];
> + default:
> + return false;
> + }
> +}
> +
> +static bool psi_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.
> + *
> + * We don't need to synchronize against CPU hotplugging. If we
> + * see a CPU that's online and has samples, we incorporate it.
> + */
> + for_each_online_cpu(cpu) {
> + struct psi_group_cpu *groupc = per_cpu_ptr(group->pcpu, cpu);
> + u32 uninitialized_var(nonidle);
urgh.. I can see why the compiler got confused. Dodgy :-)
> +
> + BUILD_BUG_ON(PSI_NONIDLE != NR_PSI_STATES - 1);
> +
> + 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.
> + delta = time - groupc->times_prev[s];
> + groupc->times_prev[s] = time;
> +
> + if (s == PSI_NONIDLE) {
> + nonidle = nsecs_to_jiffies(delta);
> + nonidle_total += nonidle;
> + } else {
> + deltas[s] += (u64)delta * nonidle;
> + }
> + }
> + }
next prev parent reply other threads:[~2018-08-03 16:57 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-01 15:19 [PATCH 0/9] psi: pressure stall information for CPU, memory, and IO v3 Johannes Weiner
2018-08-01 15:19 ` [PATCH 1/9] mm: workingset: don't drop refault information prematurely Johannes Weiner
2018-08-01 15:19 ` [PATCH 2/9] mm: workingset: tell cache transitions from workingset thrashing Johannes Weiner
2018-08-01 21:56 ` Suren Baghdasaryan
2018-08-02 12:28 ` Johannes Weiner
2018-08-01 15:19 ` [PATCH 3/9] delayacct: track delays from thrashing cache pages Johannes Weiner
2018-08-01 15:19 ` [PATCH 4/9] sched: loadavg: consolidate LOAD_INT, LOAD_FRAC, CALC_LOAD Johannes Weiner
2018-08-01 15:19 ` [PATCH 5/9] sched: loadavg: make calc_load_n() public Johannes Weiner
2018-08-01 15:19 ` [PATCH 6/9] sched: sched.h: make rq locking and clock functions available in stats.h Johannes Weiner
2018-08-01 15:19 ` [PATCH 7/9] sched: introduce this_rq_lock_irq() Johannes Weiner
2018-08-01 15:19 ` [PATCH 8/9] psi: pressure stall information for CPU, memory, and IO Johannes Weiner
2018-08-03 16:56 ` Peter Zijlstra [this message]
2018-08-06 15:05 ` Johannes Weiner
2018-08-06 15:25 ` Peter Zijlstra
2018-08-06 15:40 ` Johannes Weiner
2018-08-06 15:19 ` Johannes Weiner
2018-08-06 16:03 ` Peter Zijlstra
2018-08-21 19:44 ` Johannes Weiner
2018-08-22 9:16 ` Peter Zijlstra
2018-08-03 17:07 ` Peter Zijlstra
2018-08-06 15:23 ` Johannes Weiner
2018-08-03 17:15 ` Peter Zijlstra
2018-08-03 17:21 ` Peter Zijlstra
2018-08-21 20:11 ` Johannes Weiner
2018-08-22 9:10 ` Peter Zijlstra
2018-08-22 17:28 ` Johannes Weiner
2018-08-01 15:19 ` [PATCH 9/9] psi: cgroup support Johannes Weiner
2018-08-07 11:50 ` [PATCH 0/9] psi: pressure stall information for CPU, memory, and IO v3 peter enderborg
2018-08-07 17:51 ` Johannes Weiner
-- strict thread matches above, loose matches on Subject: below --
2018-08-28 17:22 [PATCH 0/9] psi: pressure stall information for CPU, memory, and IO v4 Johannes Weiner
2018-08-28 17:22 ` [PATCH 8/9] psi: pressure stall information for CPU, memory, and IO Johannes Weiner
2018-08-28 20:11 ` Randy Dunlap
2018-08-28 20:56 ` Johannes Weiner
2018-08-28 21:30 ` Randy Dunlap
2018-09-07 10:16 ` Peter Zijlstra
2018-09-07 10:21 ` Peter Zijlstra
2018-09-07 14:44 ` Johannes Weiner
2018-09-07 14:58 ` Peter Zijlstra
2018-09-07 17:50 ` Johannes Weiner
2018-09-07 10:24 ` Peter Zijlstra
2018-09-07 14:54 ` Johannes Weiner
2018-08-01 15:12 Johannes Weiner
2018-08-01 15:13 ` [PATCH 8/9] psi: pressure stall information for CPU, memory, and IO Johannes Weiner
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