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

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Date:   Thu, 01 Dec 2022 11:01:24 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     Tejun Heo <tj@...nel.org>
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 30/11/22 11:06, Tejun Heo wrote:
> 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.
>

That would let the timer be re-armed when the cull work is pending, which
itself will re-arm the timer to the next non-culled idle worker expiry (if
there is any remaining).

Not an issue per se, it's just that having the cull work pending is a
"promise" that the timer will be re-armed if and when necessary.

I think in cases where the cull work doesn't get to run for a while, not
having the extra work_pending() check and just arming the timer in
worker_enter_idle() might be cheaper than repeatedly checking both
timer_pending() and work_pending(), but otherwise I would assume not arming
the timer would be preferred.

>>              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.
>

Ack.

>> + *
>> + * 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).
>

Patch 4/4 adds the rest of the comment, but I can make the whole thing
appear in patch 4 if you prefer.