Re: [RFC 2/3] mm: throttle on IO only when there are too many dirty and writeback pages

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On 2015/10/30 0:17, mhocko@kernel.org wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> wait_iff_congested has been used to throttle allocator before it retried
> another round of direct reclaim to allow the writeback to make some
> progress and prevent reclaim from looping over dirty/writeback pages
> without making any progress. We used to do congestion_wait before
> 0e093d99763e ("writeback: do not sleep on the congestion queue if
> there are no congested BDIs or if significant congestion is not being
> encountered in the current zone") but that led to undesirable stalls
> and sleeping for the full timeout even when the BDI wasn't congested.
> Hence wait_iff_congested was used instead. But it seems that even
> wait_iff_congested doesn't work as expected. We might have a small file
> LRU list with all pages dirty/writeback and yet the bdi is not congested
> so this is just a cond_resched in the end and can end up triggering pre
> mature OOM.
> 
> This patch replaces the unconditional wait_iff_congested by
> congestion_wait which is executed only if we _know_ that the last round
> of direct reclaim didn't make any progress and dirty+writeback pages are
> more than a half of the reclaimable pages on the zone which might be
> usable for our target allocation. This shouldn't reintroduce stalls
> fixed by 0e093d99763e because congestion_wait is called only when we
> are getting hopeless when sleeping is a better choice than OOM with many
> pages under IO.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>   mm/page_alloc.c | 19 +++++++++++++++++--
>   1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9c0abb75ad53..0518ca6a9776 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3191,8 +3191,23 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>   		 */
>   		if (__zone_watermark_ok(zone, order, min_wmark_pages(zone),
>   				ac->high_zoneidx, alloc_flags, target)) {
> -			/* Wait for some write requests to complete then retry */
> -			wait_iff_congested(zone, BLK_RW_ASYNC, HZ/50);
> +			unsigned long writeback = zone_page_state(zone, NR_WRITEBACK),
> +				      dirty = zone_page_state(zone, NR_FILE_DIRTY);
> +
> +			if (did_some_progress)
> +				goto retry;
> +
> +			/*
> +			 * If we didn't make any progress and have a lot of
> +			 * dirty + writeback pages then we should wait for
> +			 * an IO to complete to slow down the reclaim and
> +			 * prevent from pre mature OOM
> +			 */
> +			if (2*(writeback + dirty) > reclaimable)

Doesn't this add unnecessary latency if other zones have enough clean memory ?


Thanks,
-Kame
 


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