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

[Minchan Kim <minchan.kim@gmail.com>]

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@csn.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?

-- 
Kind regards,
Minchan Kim