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SubjectRe: [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.


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