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SubjectRe: [PATCH 03/10] writeback: Do not congestion sleep if there are no congested BDIs or significant writeback
On Mon,  6 Sep 2010 11:47:26 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> If congestion_wait() is called with no BDIs congested, the caller will sleep
> for the full timeout and this may be an unnecessary sleep. This patch adds
> a wait_iff_congested() that checks congestion and only sleeps if a BDI is
> congested or if there is a significant amount of writeback going on in an
> interesting zone. Else, it calls cond_resched() to ensure the caller is
> not hogging the CPU longer than its quota but otherwise will not sleep.
>
> This is aimed at reducing some of the major desktop stalls reported during
> IO. For example, while kswapd is operating, it calls congestion_wait()
> but it could just have been reclaiming clean page cache pages with no
> congestion. Without this patch, it would sleep for a full timeout but after
> this patch, it'll just call schedule() if it has been on the CPU too long.
> Similar logic applies to direct reclaimers that are not making enough
> progress.
>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
> include/linux/backing-dev.h | 2 +-
> include/trace/events/writeback.h | 7 ++++
> mm/backing-dev.c | 66 ++++++++++++++++++++++++++++++++++++-
> mm/page_alloc.c | 4 +-
> mm/vmscan.c | 26 ++++++++++++--
> 5 files changed, 96 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index 35b0074..f1b402a 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -285,7 +285,7 @@ enum {
> void clear_bdi_congested(struct backing_dev_info *bdi, int sync);
> 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);
>
> static inline bool bdi_cap_writeback_dirty(struct backing_dev_info *bdi)
> {
> diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
> index 275d477..eeaf1f5 100644
> --- a/include/trace/events/writeback.h
> +++ b/include/trace/events/writeback.h
> @@ -181,6 +181,13 @@ DEFINE_EVENT(writeback_congest_waited_template, writeback_congestion_wait,
> TP_ARGS(usec_timeout, usec_delayed)
> );
>
> +DEFINE_EVENT(writeback_congest_waited_template, writeback_wait_iff_congested,
> +
> + TP_PROTO(unsigned int usec_timeout, unsigned int usec_delayed),
> +
> + TP_ARGS(usec_timeout, usec_delayed)
> +);
> +
> #endif /* _TRACE_WRITEBACK_H */
>
> /* This part must be outside protection */
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 298975a..94b5433 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]);
> smp_mb__after_clear_bit();
> if (waitqueue_active(wqh))
> wake_up(wqh);
> @@ -743,7 +745,8 @@ void set_bdi_congested(struct backing_dev_info *bdi, int sync)
> enum bdi_state bit;
>
> bit = sync ? BDI_sync_congested : BDI_async_congested;
> - set_bit(bit, &bdi->state);
> + if (!test_and_set_bit(bit, &bdi->state))
> + atomic_inc(&nr_bdi_congested[sync]);
> }
> EXPORT_SYMBOL(set_bdi_congested);
>
> @@ -774,3 +777,62 @@ long congestion_wait(int sync, long timeout)
> }
> EXPORT_SYMBOL(congestion_wait);
>
> +/**
> + * congestion_wait - wait for a backing_dev to become uncongested
> + * @zone: A zone to consider the number of being being written back from
> + * @sync: SYNC or ASYNC IO
> + * @timeout: timeout in jiffies
> + *
> + * Waits for up to @timeout jiffies for a backing_dev (any backing_dev) to exit
> + * write congestion. If no backing_devs are congested then the number of
> + * writeback pages in the zone are checked and compared to the inactive
> + * list. If there is no sigificant writeback or congestion, there is no point
> + * in sleeping but cond_resched() is called in case the current process has
> + * consumed its CPU quota.
> + */
> +long wait_iff_congested(struct zone *zone, int sync, long timeout)
> +{
> + long ret;
> + unsigned long start = jiffies;
> + DEFINE_WAIT(wait);
> + wait_queue_head_t *wqh = &congestion_wqh[sync];
> +
> + /*
> + * If there is no congestion, check the amount of writeback. If there
> + * is no significant writeback and no congestion, just cond_resched
> + */
> + if (atomic_read(&nr_bdi_congested[sync]) == 0) {
> + unsigned long inactive, writeback;
> +
> + inactive = zone_page_state(zone, NR_INACTIVE_FILE) +
> + zone_page_state(zone, NR_INACTIVE_ANON);
> + writeback = zone_page_state(zone, NR_WRITEBACK);
> +
> + /*
> + * If less than half the inactive list is being written back,
> + * reclaim might as well continue
> + */
> + if (writeback < inactive / 2) {

Hmm..can't we have a way that "find a page which can be just dropped without writeback"
rather than sleeping ? I think we can throttole the number of victims for avoidng I/O
congestion as pages/tick....if exhausted, ok, we should sleep.

Thanks,
-Kame







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