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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 16 Feb 2012 11:04:15 +0800
Wu Fengguang <fengguang.wu@intel.com> wrote:

> 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?
> 

Maybe. But there may be problem in looking up memcg from page at every
rotation. I think it's ok to start with a way ignoring per-memcg status.
Sorry for noise.

Thanks,
-Kame

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