linux-kernel - Re: [PATCH 08/10] psi: pressure stall information for CPU, memory, and IO

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Message-ID: <20180718124627.GD2476@hirez.programming.kicks-ass.net>
Date:   Wed, 18 Jul 2018 14:46:27 +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: [PATCH 08/10] psi: pressure stall information for CPU, memory,
 and IO

On Thu, Jul 12, 2018 at 01:29:40PM -0400, Johannes Weiner wrote:

> +static inline void psi_enqueue(struct task_struct *p, u64 now, bool wakeup)
> +{
> +	int clear = 0, set = TSK_RUNNING;
> +
> +	if (psi_disabled)
> +		return;
> +
> +	if (!wakeup || p->sched_psi_wake_requeue) {
> +		if (p->flags & PF_MEMSTALL)
> +			set |= TSK_MEMSTALL;
> +		if (p->sched_psi_wake_requeue)
> +			p->sched_psi_wake_requeue = 0;
> +	} else {
> +		if (p->in_iowait)
> +			clear |= TSK_IOWAIT;
> +	}
> +
> +	psi_task_change(p, now, clear, set);
> +}
> +
> +static inline void psi_dequeue(struct task_struct *p, u64 now, bool sleep)
> +{
> +	int clear = TSK_RUNNING, set = 0;
> +
> +	if (psi_disabled)
> +		return;
> +
> +	if (!sleep) {
> +		if (p->flags & PF_MEMSTALL)
> +			clear |= TSK_MEMSTALL;
> +	} else {
> +		if (p->in_iowait)
> +			set |= TSK_IOWAIT;
> +	}
> +
> +	psi_task_change(p, now, clear, set);
> +}

> +/**
> + * psi_memstall_enter - mark the beginning of a memory stall section
> + * @flags: flags to handle nested sections
> + *
> + * Marks the calling task as being stalled due to a lack of memory,
> + * such as waiting for a refault or performing reclaim.
> + */
> +void psi_memstall_enter(unsigned long *flags)
> +{
> +	struct rq_flags rf;
> +	struct rq *rq;
> +
> +	if (psi_disabled)
> +		return;
> +
> +	*flags = current->flags & PF_MEMSTALL;
> +	if (*flags)
> +		return;
> +	/*
> +	 * PF_MEMSTALL setting & accounting needs to be atomic wrt
> +	 * changes to the task's scheduling state, otherwise we can
> +	 * race with CPU migration.
> +	 */
> +	rq = this_rq_lock_irq(&rf);
> +
> +	update_rq_clock(rq);
> +
> +	current->flags |= PF_MEMSTALL;
> +	psi_task_change(current, rq_clock(rq), 0, TSK_MEMSTALL);
> +
> +	rq_unlock_irq(rq, &rf);
> +}

I'm confused by this whole MEMSTALL thing... I thought the idea was to
account the time we were _blocked_ because of memstall, but you seem to
count the time we're _running_ with PF_MEMSTALL.


And esp. the wait_on_page_bit_common caller seems performance sensitive,
and the above function is quite expensive.