Re: reclaim the LRU lists full of dirty/writeback pages

[Wu Fengguang <fengguang.wu@intel.com>]

On Thu, Feb 16, 2012 at 09:00:37AM +0900, KAMEZAWA Hiroyuki wrote:
> On Tue, 14 Feb 2012 21:18:12 +0800
> Wu Fengguang <fengguang.wu@intel.com> wrote:
> 
> > 
> > --- linux.orig/include/linux/backing-dev.h	2012-02-14 19:43:06.000000000 +0800
> > +++ linux/include/linux/backing-dev.h	2012-02-14 19:49:26.000000000 +0800
> > @@ -304,6 +304,8 @@ void clear_bdi_congested(struct backing_
> >  void set_bdi_congested(struct backing_dev_info *bdi, int sync);
> >  long congestion_wait(int sync, long timeout);
> >  long wait_iff_congested(struct zone *zone, int sync, long timeout);
> > +long reclaim_wait(long timeout);
> > +void reclaim_rotated(void);
> >  
> >  static inline bool bdi_cap_writeback_dirty(struct backing_dev_info *bdi)
> >  {
> > --- linux.orig/mm/backing-dev.c	2012-02-14 19:26:15.000000000 +0800
> > +++ linux/mm/backing-dev.c	2012-02-14 20:09:45.000000000 +0800
> > @@ -873,3 +873,38 @@ out:
> >  	return ret;
> >  }
> >  EXPORT_SYMBOL(wait_iff_congested);
> > +
> > +static DECLARE_WAIT_QUEUE_HEAD(reclaim_wqh);
> > +
> > +/**
> > + * reclaim_wait - wait for some pages being rotated to the LRU tail
> > + * @timeout: timeout in jiffies
> > + *
> > + * Wait until @timeout, or when some (typically PG_reclaim under writeback)
> > + * pages rotated to the LRU so that page reclaim can make progress.
> > + */
> > +long reclaim_wait(long timeout)
> > +{
> > +	long ret;
> > +	unsigned long start = jiffies;
> > +	DEFINE_WAIT(wait);
> > +
> > +	prepare_to_wait(&reclaim_wqh, &wait, TASK_KILLABLE);
> > +	ret = io_schedule_timeout(timeout);
> > +	finish_wait(&reclaim_wqh, &wait);
> > +
> > +	trace_writeback_reclaim_wait(jiffies_to_usecs(timeout),
> > +				     jiffies_to_usecs(jiffies - start));
> > +
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL(reclaim_wait);
> > +
> > +void reclaim_rotated()
> > +{
> > +	wait_queue_head_t *wqh = &reclaim_wqh;
> > +
> > +	if (waitqueue_active(wqh))
> > +		wake_up(wqh);
> > +}
> > +
> 
> Thank you.
> 
> I like this approach. A nitpick is that this may wake up all waiters 
> in the system when a memcg is rotated.

Thank you. It sure helps to start it simple :-)

> How about wait_event() + condition by bitmap (using per memcg unique IDs.) ?

I'm not sure how to manage the bitmap. The idea in my mind is to

- maintain a memcg->pages_rotated counter

- in reclaim_wait(), grab the current ->pages_rotated value before
  going to wait, compare it to the new value on every wakeup, and
  return to the user when seeing a different ->pages_rotated value.
  (this cannot stop waking up multiple tasks in the same memcg...) 

Does that sound reasonable?

Thanks,
Fengguang

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>