linux-kernel - Re: [PATCH 1/2] sched/wait: Break up long wake list walk

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Date:   Fri, 18 Aug 2017 09:55:56 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     "Liang, Kan" <kan.liang@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/wait: Break up long wake list walk

On Fri, Aug 18, 2017 at 5:23 AM, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
>         new_page = alloc_pages_node(node,
> -               (GFP_TRANSHUGE_LIGHT | __GFP_THISNODE),
> +               (GFP_TRANSHUGE_LIGHT | __GFP_THISNODE) & ~__GFP_DIRECT_RECLAIM,
>                 HPAGE_PMD_ORDER);

That can't make any difference. We already have:

  #define GFP_TRANSHUGE_LIGHT     ((GFP_HIGHUSER_MOVABLE | __GFP_COMP | \
                         __GFP_NOMEMALLOC | __GFP_NOWARN) & ~__GFP_RECLAIM)

and that "& ~__GFP_RECLAIM" is removing __GFP_DIRECT_RECLAIM.

So that patch is a no-op, afaik.

Is there something else expensive in there?

It *might* be simply that we have a shit-ton of threads, and the
thread that holds the page lock for migration is just preempted out.
even if it doesn't really do anything particularly expensive.

                           Linus