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?
--
Kind regards,
Minchan Kim
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
