Re: [PATCH 05/10] writeback: per task dirty rate limit

Wu Fengguang
2011-08-26T12:59:31Z

On Fri, Aug 26, 2011 at 08:51:09PM +0800, Peter Zijlstra wrote:
> On Fri, 2011-08-26 at 19:38 +0800, Wu Fengguang wrote:
> > + preempt_disable();
> > /*
> > - * Check the rate limiting. Also, we do not want to throttle real-time
> > - * tasks in balance_dirty_pages(). Period.
> > + * This prevents one CPU to accumulate too many dirtied pages without
> > + * calling into balance_dirty_pages(), which can happen when there are
> > + * 1000+ tasks, all of them start dirtying pages at exactly the same
> > + * time, hence all honoured too large initial task->nr_dirtied_pause.
> > */
> > - preempt_disable();
> > p = &__get_cpu_var(bdp_ratelimits);
>
> p = &get_cpu_var(bdp_ratelimits);

Ah yeah.. I actually followed your suggestion, and then find we'll
eventually do two __get_cpu_var() calls here, one for bdp_ratelimits
and the other for dirty_leaks in a planned patch. So let's keep the
preempt_disable()/preempt_enable().

> > - *p += nr_pages_dirtied;
> > - if (unlikely(*p >= ratelimit)) {
> > - ratelimit = sync_writeback_pages(*p);
> > + if (unlikely(current->nr_dirtied >= ratelimit))
> > *p = 0;
> > - preempt_enable();
> > - balance_dirty_pages(mapping, ratelimit);
> > - return;
> > + else {
> > + *p += nr_pages_dirtied;
> > + if (unlikely(*p >= ratelimit_pages)) {
> > + *p = 0;
> > + ratelimit = 0;
> > + }
> > }
> > preempt_enable();
>
> put_cpu_var(bdp_ratelimits);

ditto.

Thanks,
Fengguang