linux-kernel - Re: question: should_compact

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Message-ID: <908c1454-6ae5-87ca-c6a5-e542fbafa866@suse.cz>
Date:   Wed, 5 Jun 2019 09:58:49 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Cc:     Michal Hocko <mhocko@...nel.org>
Subject: Re: question: should_compact_retry limit

On 6/5/19 1:30 AM, Mike Kravetz wrote:
> While looking at some really long hugetlb page allocation times, I noticed
> instances where should_compact_retry() was returning true more often that
> I expected.  In one allocation attempt, it returned true 765668 times in a
> row.  To me, this was unexpected because of the following:
> 
> #define MAX_COMPACT_RETRIES 16
> int max_retries = MAX_COMPACT_RETRIES;
> 
> However, if should_compact_retry() returns true via the following path we
> do not increase the retry count.
> 
> 	/*
> 	 * make sure the compaction wasn't deferred or didn't bail out early
> 	 * due to locks contention before we declare that we should give up.
> 	 * But do not retry if the given zonelist is not suitable for
> 	 * compaction.
> 	 */
> 	if (compaction_withdrawn(compact_result)) {
> 		ret = compaction_zonelist_suitable(ac, order, alloc_flags);
> 		goto out;
> 	}
> 
> Just curious, is this intentional?

Hmm I guess we didn't expect compaction_withdrawn() to be so
consistently returned. Do you know what value of compact_result is there
in your test?