linux-kernel - Re: [RFC PATCH 10/10] psi: aggregate ongoing stall events when somebody reads pressure

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Date:   Tue, 17 Jul 2018 17:13:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Christopher Lameter <cl@...ux.com>,
        Mike Galbraith <efault@....de>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [RFC PATCH 10/10] psi: aggregate ongoing stall events when
 somebody reads pressure

On Thu, Jul 12, 2018 at 01:29:42PM -0400, Johannes Weiner wrote:
> @@ -218,10 +216,36 @@ 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;
> +		struct rq_flags rf;
> +		struct rq *rq;
> +		u64 now;
>  
> -		if (!groupc->nonidle_time)
> +		if (!groupc->nonidle_time && !groupc->nonidle)
>  			continue;
>  
> +		/*
> +		 * We come here for two things: 1) periodic per-cpu
> +		 * bucket flushing and averaging and 2) when the user
> +		 * wants to read a pressure file. For flushing and
> +		 * averaging, which is relatively infrequent, we can
> +		 * be lazy and tolerate some raciness with concurrent
> +		 * updates to the per-cpu counters. However, if a user
> +		 * polls the pressure state, we want to give them the
> +		 * most uptodate information we have, including any
> +		 * currently active state which hasn't been timed yet,
> +		 * because in case of an iowait or a reclaim run, that
> +		 * can be significant.
> +		 */
> +		if (ondemand) {
> +			rq = cpu_rq(cpu);
> +			rq_lock_irq(rq, &rf);

That's a DoS right there..