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@xxxxxxxxx>
> > ---
> > 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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