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

[Jens Axboe <jens.axboe@oracle.com>]

On Mon, Jul 06 2009, Artem Bityutskiy wrote:
> 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).

That is correct. Note that these exit when they have been idle for a
while, for embedded and such you could make it more aggressive by
exiting quicker. The sync_supers should be directly fixable by your
sb_dirty() stuff.

So I don't think it's a huge change from what we currently have.

> 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?

You could replace the schedule_timeout() by a schedule(), and instead
have a single timer running that would scan the bdi_list and issue the
kupdated() timed writeback that is the reason it uses schedule_timeout()
now. Explicitly issued work will manually wake up the per-bdi thread(s).
That single timer could easily handle waking up bdi_sync_supers() as
well.

-- 
Jens Axboe