linux-kernel - Re: [PATCH 6/7] psi: pressure stall information for CPU, memory, and IO

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Date:   Wed, 9 May 2018 12:21:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org, cgroups@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Mike Galbraith <efault@....de>,
        Oliver Yang <yangoliver@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        xxx xxx <x.qendo@...il.com>,
        Taras Kondratiuk <takondra@...co.com>,
        Daniel Walker <danielwa@...co.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Ruslan Ruslichenko <rruslich@...co.com>, kernel-team@...com
Subject: Re: [PATCH 6/7] psi: pressure stall information for CPU, memory, and
 IO

On Mon, May 07, 2018 at 05:01:34PM -0400, Johannes Weiner wrote:
> +/**
> + * 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;
> +
> +	*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.
> +	 */
> +	local_irq_disable();
> +	rq = this_rq();
> +	raw_spin_lock(&rq->lock);
> +	rq_pin_lock(rq, &rf);

Given that churn in sched.h, you seen rq_lock() and friends.

Either write this like:

	local_irq_disable();
	rq = this_rq();
	rq_lock(rq, &rf);

Or instroduce "rq = this_rq_lock_irq()", which we could also use in
do_sched_yield().

> +	update_rq_clock(rq);
> +
> +	current->flags |= PF_MEMSTALL;
> +	psi_task_change(current, rq_clock(rq), 0, TSK_MEMSTALL);
> +
> +	rq_unpin_lock(rq, &rf);
> +	raw_spin_unlock(&rq->lock);
> +	local_irq_enable();

That's called rq_unlock_irq().

> +}
> +
> +/**
> + * psi_memstall_leave - mark the end of an memory stall section
> + * @flags: flags to handle nested memdelay sections
> + *
> + * Marks the calling task as no longer stalled due to lack of memory.
> + */
> +void psi_memstall_leave(unsigned long *flags)
> +{
> +	struct rq_flags rf;
> +	struct rq *rq;
> +
> +	if (*flags)
> +		return;
> +	/*
> +	 * PF_MEMSTALL clearing & accounting needs to be atomic wrt
> +	 * changes to the task's scheduling state, otherwise we could
> +	 * race with CPU migration.
> +	 */
> +	local_irq_disable();
> +	rq = this_rq();
> +	raw_spin_lock(&rq->lock);
> +	rq_pin_lock(rq, &rf);
> +
> +	update_rq_clock(rq);
> +
> +	current->flags &= ~PF_MEMSTALL;
> +	psi_task_change(current, rq_clock(rq), TSK_MEMSTALL, 0);
> +
> +	rq_unpin_lock(rq, &rf);
> +	raw_spin_unlock(&rq->lock);
> +	local_irq_enable();
> +}

Idem.