linux-kernel - Re: [PATCH 2/3] writeback: Record if the congestion was unnecessary

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Date:	Thu, 26 Aug 2010 18:41:06 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] writeback: Record if the congestion was unnecessary

On Fri, Aug 27, 2010 at 02:35:34AM +0900, Minchan Kim wrote:
> On Thu, Aug 26, 2010 at 04:14:15PM +0100, Mel Gorman wrote:
> > If congestion_wait() is called when there is no congestion, the caller
> > will wait for the full timeout. This can cause unreasonable and
> > unnecessary stalls. There are a number of potential modifications that
> > could be made to wake sleepers but this patch measures how serious the
> > problem is. It keeps count of how many congested BDIs there are. If
> > congestion_wait() is called with no BDIs congested, the tracepoint will
> > record that the wait was unnecessary.
> > 
> > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > ---
> >  include/trace/events/writeback.h |   11 ++++++++---
> >  mm/backing-dev.c                 |   15 ++++++++++++---
> >  2 files changed, 20 insertions(+), 6 deletions(-)
> > 
> > diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> > index e3bee61..03bb04b 100644
> > --- a/include/trace/events/writeback.h
> > +++ b/include/trace/events/writeback.h
> > @@ -155,19 +155,24 @@ DEFINE_WBC_EVENT(wbc_writepage);
> >  
> >  TRACE_EVENT(writeback_congest_waited,
> >  
> > -	TP_PROTO(unsigned int usec_delayed),
> > +	TP_PROTO(unsigned int usec_delayed, bool unnecessary),
> >  
> > -	TP_ARGS(usec_delayed),
> > +	TP_ARGS(usec_delayed, unnecessary),
> >  
> >  	TP_STRUCT__entry(
> >  		__field(	unsigned int,	usec_delayed	)
> > +		__field(	unsigned int,	unnecessary	)
> >  	),
> >  
> >  	TP_fast_assign(
> >  		__entry->usec_delayed	= usec_delayed;
> > +		__entry->unnecessary	= unnecessary;
> >  	),
> >  
> > -	TP_printk("usec_delayed=%u", __entry->usec_delayed)
> > +	TP_printk("usec_delayed=%u unnecessary=%d",
> > +		__entry->usec_delayed,
> > +		__entry->unnecessary
> > +	)
> >  );
> >  
> >  #endif /* _TRACE_WRITEBACK_H */
> > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> > index 7ae33e2..a49167f 100644
> > --- a/mm/backing-dev.c
> > +++ b/mm/backing-dev.c
> > @@ -724,6 +724,7 @@ static wait_queue_head_t congestion_wqh[2] = {
> >  		__WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[0]),
> >  		__WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[1])
> >  	};
> > +static atomic_t nr_bdi_congested[2];
> >  
> >  void clear_bdi_congested(struct backing_dev_info *bdi, int sync)
> >  {
> > @@ -731,7 +732,8 @@ void clear_bdi_congested(struct backing_dev_info *bdi, int sync)
> >  	wait_queue_head_t *wqh = &congestion_wqh[sync];
> >  
> >  	bit = sync ? BDI_sync_congested : BDI_async_congested;
> > -	clear_bit(bit, &bdi->state);
> > +	if (test_and_clear_bit(bit, &bdi->state))
> > +		atomic_dec(&nr_bdi_congested[sync]);
> 
> Hmm.. Now congestion_wait's semantics "wait for _a_ backing_dev to become uncongested"
> But this seems to consider whole backing dev. Is your intention? or Am I missing now?
> 

Not whole backing devs, all backing devs. This is intentional.

If congestion_wait() is called with 0 BDIs congested, we sleep the full timeout
because a wakeup event will not occur - this is a bad scenario. To know if
0 BDIs were congested, one could either walk all the BDIs checking their
status or maintain a counter like nr_bdi_congested which is what I decided on.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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