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Date:   Fri, 20 Jul 2018 22:35:24 +0200
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>,
        Vinayak Menon <vinmenon@codeaurora.org>,
        Christopher Lameter <cl@linux.com>,
        Mike Galbraith <efault@gmx.de>,
        Shakeel Butt <shakeelb@google.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH 08/10] psi: pressure stall information for CPU, memory,
 and IO
Message-ID: <20180720203524.GD4920@worktop.programming.kicks-ass.net>
References: <20180712172942.10094-1-hannes@cmpxchg.org>
 <20180712172942.10094-9-hannes@cmpxchg.org>
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On Thu, Jul 12, 2018 at 01:29:40PM -0400, Johannes Weiner wrote:
> +static bool psi_update_stats(struct psi_group *group)
> +{

> +	for_each_online_cpu(cpu) {
> +		struct psi_group_cpu *groupc = per_cpu_ptr(group->cpus, cpu);
> +		unsigned long nonidle;
> +
> +		if (!groupc->nonidle_time)
> +			continue;
> +
> +		nonidle = nsecs_to_jiffies(groupc->nonidle_time);
> +		groupc->nonidle_time = 0;
> +		nonidle_total += nonidle;
> +
> +		for (r = 0; r < NR_PSI_RESOURCES; r++) {
> +			struct psi_resource *res = &groupc->res[r];
> +
> +			some[r] += (res->times[0] + res->times[1]) * nonidle;
> +			full[r] += res->times[1] * nonidle;
> +
> +			/* It's racy, but we can tolerate some error */
> +			res->times[0] = 0;
> +			res->times[1] = 0;
> +		}
> +	}

An alternative for this, that also allows that ondemand update, but
without spamming the rq->lock would be something like:

struct psi_group_cpu {
	u32 tasks[3];
	u32 cpu_state : 2;
	u32 mem_state : 2;
	u32 io_state  : 2;
	u32 :0;

	u64 last_update_time;

	u32 nonidle;
	u32 full[2];
	u32 some[3];
} ____cacheline_aligned_in_smp;

/* Allocate _2_ copies */
DEFINE_PER_CPU_ALIGNED_SHARED(struct psi_group_cpu[2], psi_cpus);

struct psi_group global_psi = {
	.cpus = &psi_cpus[0],
};


	u64 sums[6] = { 0, };

	for_each_possible_cpu(cpu) {
		struct psi_group_cpu *pgc = per_cpu_ptr(group->cpus, cpu);
		u32 *active, *shadow;

		active = &pgc[0].nonidle;
		shadow = &pgc[1].nonidle;

		/*
		 * Compare the active count to the shadow count
		 * if different, compute the delta and update the shadow
		 * copy.
		 * This only writes to the shadow copy (separate line)
		 * and leaves the active a read-only access.
		 */
		for (i = 0; i < 6; i++) {
			u32 old = READ_ONCE(shadow[i]);
			u32 new = READ_ONCE(active[i]);

			delta = (new - old);
			if (!delta) {
				if (!i)
					goto next;
				continue;
			}

			WRITE_ONCE(shadow[i], new);

			sums[i] += delta;
		}
next:		;
	}