linux-kernel - Re: [PATCH v6 3/4] workqueue: Convert the idle_timer to a timer + work

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Date:   Wed, 30 Nov 2022 11:06:20 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Phil Auld <pauld@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH v6 3/4] workqueue: Convert the idle_timer to a timer +
 work_struct

On Mon, Nov 28, 2022 at 06:31:08PM +0000, Valentin Schneider wrote:
> @@ -1806,7 +1808,9 @@ static void worker_enter_idle(struct worker *worker)
>  	/* idle_list is LIFO */
>  	list_add(&worker->entry, &pool->idle_list);
>  
> -	if (too_many_workers(pool) && !timer_pending(&pool->idle_timer))
> +	if (too_many_workers(pool) &&
> +	    !timer_pending(&pool->idle_timer) &&
> +	    !work_pending(&pool->idle_cull_work))

Just checking the timer should be enough here, I think.

>  		mod_timer(&pool->idle_timer, jiffies + IDLE_WORKER_TIMEOUT);
>  
>  	/* Sanity check nr_running. */
> @@ -2019,17 +2023,56 @@ static void destroy_worker(struct worker *worker)
>  	wake_up_process(worker->task);
>  }
>  
> +/*
> + * idle_worker_timeout - check if some idle workers can now be deleted.

Might as well turn it into a proper function comment starting w/ "/**" and
with argument list.

> + *
> + * The timer is armed in worker_enter_idle(). Note that it isn't disarmed in
> + * worker_leave_idle(), as a worker flicking between idle and active while its
> + * pool is at the too_many_workers() tipping point would cause too much timer
> + * housekeeping overhead. Since IDLE_WORKER_TIMEOUT is long enough, we just let
> + * it expire and re-evaluate things from there.
> + */
>  static void idle_worker_timeout(struct timer_list *t)
>  {
>  	struct worker_pool *pool = from_timer(pool, t, idle_timer);
> +	bool do_cull = false;
> +
> +	if (work_pending(&pool->idle_cull_work))
> +		return;
>  
>  	raw_spin_lock_irq(&pool->lock);
>  
> -	while (too_many_workers(pool)) {
> +	if (too_many_workers(pool)) {
>  		struct worker *worker;
>  		unsigned long expires;
>  
>  		/* idle_list is kept in LIFO order, check the last one */
> +		worker = list_entry(pool->idle_list.prev, struct worker, entry);
> +		expires = worker->last_active + IDLE_WORKER_TIMEOUT;
> +		do_cull = !time_before(jiffies, expires);
> +
> +		if (!do_cull)
> +			mod_timer(&pool->idle_timer, expires);
> +	}
> +	raw_spin_unlock_irq(&pool->lock);
> +
> +	if (do_cull)
> +		queue_work(system_unbound_wq, &pool->idle_cull_work);
> +}
> +
> +/*
> + * idle_cull_fn - cull workers that have been idle for too long.
> + */

Please turn it into a full function comment or drop the wings (ie. make it
an one-liner).

> +static void idle_cull_fn(struct work_struct *work)
> +{
> +	struct worker_pool *pool = container_of(work, struct worker_pool, idle_cull_work);
> +
> +	raw_spin_lock_irq(&pool->lock);
> +
> +	while (too_many_workers(pool)) {
> +		struct worker *worker;
> +		unsigned long expires;
> +

Other than that, looks great to me.

Thanks.

-- 
tejun