On Thu, Feb 16, 2012 at 12:52:21PM +0900, KAMEZAWA Hiroyuki wrote:
> On Thu, 16 Feb 2012 11:04:15 +0800
> Wu Fengguang <fengguang.wu@xxxxxxxxx> 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@xxxxxxxxx> 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.
When there comes such a need, we could take some sampled approach: for
each pagevec rotated, only dereference one of its pages and wakeup the
corresponding memcg task(s). That should do well enough in practice
even for random write patterns.
Thanks,
Fengguang
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