linux-kernel - Re: [PATCH 9/9] mm: remove unnecessary back-off function when retrying page reclaim

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Message-ID: <20170301145656.GA11730@dhcp22.suse.cz>
Date:   Wed, 1 Mar 2017 15:56:56 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jia He <hejianet@...il.com>, Mel Gorman <mgorman@...e.de>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 9/9] mm: remove unnecessary back-off function when
 retrying page reclaim

On Tue 28-02-17 16:40:07, Johannes Weiner wrote:
> The backoff mechanism is not needed. If we have MAX_RECLAIM_RETRIES
> loops without progress, we'll OOM anyway; backing off might cut one or
> two iterations off that in the rare OOM case. If we have intermittent
> success reclaiming a few pages, the backoff function gets reset also,
> and so is of little help in these scenarios.

Yes, as already mentioned elsewhere the original intention was to a more
graceful oom convergence when we are trashing over last few reclaimable
pages but as the code evolved the result is not all that great.
 
> We might want a backoff function for when there IS progress, but not
> enough to be satisfactory. But this isn't that. Remove it.

Completely agreed.
 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/page_alloc.c | 15 ++++++---------
>  1 file changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9ac639864bed..223644afed28 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3511,11 +3511,10 @@ bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
>  /*
>   * Checks whether it makes sense to retry the reclaim to make a forward progress
>   * for the given allocation request.
> - * The reclaim feedback represented by did_some_progress (any progress during
> - * the last reclaim round) and no_progress_loops (number of reclaim rounds without
> - * any progress in a row) is considered as well as the reclaimable pages on the
> - * applicable zone list (with a backoff mechanism which is a function of
> - * no_progress_loops).
> + *
> + * We give up when we either have tried MAX_RECLAIM_RETRIES in a row
> + * without success, or when we couldn't even meet the watermark if we
> + * reclaimed all remaining pages on the LRU lists.
>   *
>   * Returns true if a retry is viable or false to enter the oom path.
>   */
> @@ -3560,13 +3559,11 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
>  		bool wmark;
>  
>  		available = reclaimable = zone_reclaimable_pages(zone);
> -		available -= DIV_ROUND_UP((*no_progress_loops) * available,
> -					  MAX_RECLAIM_RETRIES);
>  		available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
>  
>  		/*
> -		 * Would the allocation succeed if we reclaimed the whole
> -		 * available?
> +		 * Would the allocation succeed if we reclaimed all
> +		 * reclaimable pages?
>  		 */
>  		wmark = __zone_watermark_ok(zone, order, min_wmark,
>  				ac_classzone_idx(ac), alloc_flags, available);
> -- 
> 2.11.1

-- 
Michal Hocko
SUSE Labs