linux-kernel - Re: [PATCH 2/5] mm: compaction: simplify should_compact

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Date:   Mon, 29 May 2023 12:38:05 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 2/5] mm: compaction: simplify should_compact_retry()

On Mon, May 29, 2023 at 03:03:52PM +0200, Vlastimil Babka wrote:
> On 5/19/23 14:39, Johannes Weiner wrote:
> > The different branches for retry are unnecessarily complicated. There
> > are really only three outcomes: progress (retry n times), skipped
> > (retry if reclaim can help), failed (retry with higher priority).
> > 
> > Rearrange the branches and the retry counter to make it simpler.
> > 
> > v2:
> > - fix trace point build (Mel)
> > - fix max_retries logic for costly allocs (Huang)
> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > ---
> >  mm/page_alloc.c | 53 +++++++++++++++----------------------------------
> >  1 file changed, 16 insertions(+), 37 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 5a84a0bebc37..72660e924b95 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3772,16 +3772,22 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
> >  	 * Compaction managed to coalesce some page blocks, but the
> >  	 * allocation failed presumably due to a race. Retry some.
> >  	 */
> > -	if (compact_result == COMPACT_SUCCESS)
> > -		(*compaction_retries)++;
> > +	if (compact_result == COMPACT_SUCCESS) {
> > +		/*
> > +		 * !costly requests are much more important than
> > +		 * __GFP_RETRY_MAYFAIL costly ones because they are de
> > +		 * facto nofail and invoke OOM killer to move on while
> > +		 * costly can fail and users are ready to cope with
> > +		 * that. 1/4 retries is rather arbitrary but we would
> > +		 * need much more detailed feedback from compaction to
> > +		 * make a better decision.
> > +		 */
> > +		if (order > PAGE_ALLOC_COSTLY_ORDER)
> > +			max_retries /= 4;
> >  
> > -	/*
> > -	 * All zones were scanned completely and still no result. It
> > -	 * doesn't really make much sense to retry except when the
> > -	 * failure could be caused by insufficient priority
> > -	 */
> > -	if (compact_result == COMPACT_COMPLETE)
> > -		goto check_priority;
> > +		ret = ++(*compaction_retries) <= max_retries;
> > +		goto out;
> 
> I think you simplified this part too much, so now once it runs out of
> retries, it will return false, while previously it would increase the priority.

Oops, I'll send a delta fix to Andrew tomorrow. Thanks!