linux-kernel - question: should_compact

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Message-ID: <6377c199-2b9e-e30d-a068-c304d8a3f706@oracle.com>
Date:   Tue, 4 Jun 2019 16:30:47 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...nel.org>
Subject: question: should_compact_retry limit

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?
-- 
Mike Kravetz