Re: [PATCH 3/5] page allocator: Wait on both sync and async congestion after direct reclaim

KOSAKI Motohiro
2009-11-13T11:20:41Z

(cc to Jens)

> Testing by Frans Pop indicated that in the 2.6.30..2.6.31 window at least
> that the commits 373c0a7e 8aa7e847 dramatically increased the number of
> GFP_ATOMIC failures that were occuring within a wireless driver. Reverting
> this patch seemed to help a lot even though it was pointed out that the
> congestion changes were very far away from high-order atomic allocations.
>
> The key to why the revert makes such a big difference is down to timing and
> how long direct reclaimers wait versus kswapd. With the patch reverted,
> the congestion_wait() is on the SYNC queue instead of the ASYNC. As a
> significant part of the workload involved reads, it makes sense that the
> SYNC list is what was truely congested and with the revert processes were
> waiting on congestion as expected. Hence, direct reclaimers stalled
> properly and kswapd was able to do its job with fewer stalls.
>
> This patch aims to fix the congestion_wait() behaviour for SYNC and ASYNC
> for direct reclaimers. Instead of making the congestion_wait() on the SYNC
> queue which would only fix a particular type of workload, this patch adds a
> third type of congestion_wait - BLK_RW_BOTH which first waits on the ASYNC
> and then the SYNC queue if the timeout has not been reached. In tests, this
> counter-intuitively results in kswapd stalling less and freeing up pages
> resulting in fewer allocation failures and fewer direct-reclaim-orientated
> stalls.

Honestly, I don't like this patch. page allocator is not related to
sync block queue. vmscan doesn't make read operation.
This patch makes nearly same effect of s/congestion_wait/io_schedule_timeout/.

Please don't make mysterious heuristic code.

Sidenode: I doubt this regression was caused from page allocator.
Probably we need to confirm caller change....

>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
> include/linux/backing-dev.h | 1 +
> mm/backing-dev.c | 25 ++++++++++++++++++++++---
> mm/page_alloc.c | 4 ++--
> mm/vmscan.c | 2 +-
> 4 files changed, 26 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index b449e73..b35344c 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -276,6 +276,7 @@ static inline int bdi_rw_congested(struct backing_dev_info *bdi)
> enum {
> BLK_RW_ASYNC = 0,
> BLK_RW_SYNC = 1,
> + BLK_RW_BOTH = 2,
> };
>
> void clear_bdi_congested(struct backing_dev_info *bdi, int sync);
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 1065b71..ea9ffc3 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -736,22 +736,41 @@ EXPORT_SYMBOL(set_bdi_congested);
>
> /**
> * congestion_wait - wait for a backing_dev to become uncongested
> - * @sync: SYNC or ASYNC IO
> + * @sync: SYNC, ASYNC or BOTH 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 just wait for the
> * next write to be completed.
> */
> -long congestion_wait(int sync, long timeout)
> +long congestion_wait(int sync_request, long timeout)
> {
> long ret;
> DEFINE_WAIT(wait);
> - wait_queue_head_t *wqh = &congestion_wqh[sync];
> + int sync;
> + wait_queue_head_t *wqh;
> +
> + /* If requested to sync both, wait on ASYNC first, then SYNC */
> + if (sync_request == BLK_RW_BOTH)
> + sync = BLK_RW_ASYNC;
> + else
> + sync = sync_request;
> +
> +again:
> + wqh = &congestion_wqh[sync];
>
> prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
> ret = io_schedule_timeout(timeout);
> finish_wait(wqh, &wait);
> +
> + if (sync_request == BLK_RW_BOTH) {
> + sync_request = 0;
> + sync = BLK_RW_SYNC;
> + timeout = ret;
> + if (timeout)
> + goto again;
> + }
> +
> return ret;
> }
> EXPORT_SYMBOL(congestion_wait);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2bc2ac6..f6ed41c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1727,7 +1727,7 @@ __alloc_pages_high_priority(gfp_t gfp_mask, unsigned int order,
> preferred_zone, migratetype);
>
> if (!page && gfp_mask & __GFP_NOFAIL)
> - congestion_wait(BLK_RW_ASYNC, HZ/50);
> + congestion_wait(BLK_RW_BOTH, HZ/50);
> } while (!page && (gfp_mask & __GFP_NOFAIL));
>
> return page;
> @@ -1898,7 +1898,7 @@ rebalance:
> pages_reclaimed += did_some_progress;
> if (should_alloc_retry(gfp_mask, order, pages_reclaimed)) {
> /* Wait for some write requests to complete then retry */
> - congestion_wait(BLK_RW_ASYNC, HZ/50);
> + congestion_wait(BLK_RW_BOTH, HZ/50);
> goto rebalance;
> }
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 777af57..190bae1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1793,7 +1793,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>
> /* Take a nap, wait for some writeback to complete */
> if (sc->nr_scanned && priority < DEF_PRIORITY - 2)
> - congestion_wait(BLK_RW_ASYNC, HZ/10);
> + congestion_wait(BLK_RW_BOTH, HZ/10);
> }
> /* top priority shrink_zones still had more to do? don't OOM, then */
> if (!sc->all_unreclaimable && scanning_global_lru(sc))
> --
> 1.6.5