Re: [PATCH 02/10] writeback: switch to per-bdi threads for flushing data

[Artem Bityutskiy <dedekind1@gmail.com>]

Jens Axboe wrote:
> +/*
> + * kupdated() used to do this. We cannot do it from the bdi_forker_task()
> + * or we risk deadlocking on ->s_umount. The longer term solution would be
> + * to implement sync_supers_bdi() or similar and simply do it from the
> + * bdi writeback tasks individually.
> + */
> +static int bdi_sync_supers(void *unused)
> +{
> +	set_user_nice(current, 0);
> +
> +	while (!kthread_should_stop()) {
> +		set_current_state(TASK_INTERRUPTIBLE);
> +		schedule();
> +
> +		/*
> +		 * Do this periodically, like kupdated() did before.
> +		 */
> +		sync_supers();
> +	}
> +
> +	return 0;

ATM we have one timer for both data and super-block synchronization.
With per-bdi write-back we have:

1. one timer for super blocks
2. many per-bdi timers for data (schedule_timeout() is essentially
   using timers).

This is not nice, because each timer is an additional source of
power-savings killers. I mean, it is more power management (PM)
friendly to have less timers and disturb CPU less, make CPU wake
up from retention less frequently.

I do not challange the per-bdi idea at all, but is it possible to
think about a more PM-friendly desing and have one source of
periodic write-back, not many. I mean, could there be one timer
which periodically syncs supers and wakes up the BDI write-back
tasks?

I've just started looking at your work, so I do not have good overall
picture of what's going on, so apologies in advance if I missed
something.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)