Re: [PATCH 03/10] writeback: Do not congestion sleep if there areno congested BDIs or significant writeback

[KAMEZAWA Hiroyuki]

On Mon,  6 Sep 2010 11:47:26 +0100
Mel Gorman <mel@xxxxxxxxx> 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@xxxxxxxxx>
> ---
>  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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