linux-kernel - Re: [PATCH 1/2] mm: thp: relax __GFP_THISNODE for MADV

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Date:   Wed, 10 Oct 2018 09:54:32 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrea Arcangeli <aarcange@...hat.com>,
        David Rientjes <rientjes@...gle.com>
Cc:     Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Argangeli <andrea@...nel.org>,
        Zi Yan <zi.yan@...rutgers.edu>,
        Stefan Priebe - Profihost AG <s.priebe@...fihost.ag>,
        "Kirill A. Shutemov" <kirill@...temov.name>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Stable tree <stable@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 1/2] mm: thp: relax __GFP_THISNODE for MADV_HUGEPAGE
 mappings

On 10/10/18 12:51 AM, Andrea Arcangeli wrote:
> Yes there's one case where reclaim is "pointless", but it happens once
> and then COMPACT_DEFERRED is returned and __GFP_NORETRY will skip
> reclaim then.
> 
> So you're right when we hit fragmentation there's one and only one
> "pointless" reclaim invocation. And immediately after we also
> exponentially backoff on the compaction invocations with the
> compaction deferred logic.
> 
> We could try optimize away such "pointless" reclaim event for sure,
> but it's probably an optimization that may just get lost in the noise
> and may not be measurable, because it only happens once when the first
> full fragmentation is encountered.

Note there's a small catch in the above. defer_compaction() has always
only been called after a failure on higher priority than
COMPACT_PRIO_ASYNC, where it's assumed that async compaction can
terminate prematurely due to a number of reasons, so it doesn't mean
that the zone itself cannot be compacted.
And, for __GFP_NORETRY, if the initial compaction fails, we keep using
async compaction also for the second, after-reclaim attempt (which would
otherwise use SYNC_LIGHT):

        /*
         * Looks like reclaim/compaction is worth trying, but
         * sync compaction could be very expensive, so keep
         * using async compaction.
         */
        compact_priority = INIT_COMPACT_PRIORITY;

This doesn't affect current madvised THP allocation which doesn't use
__GFP_NORETRY, but could explain why you saw no benefit from changing it
to __GFP_NORETRY.