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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 23 Aug 2017 11:17:30 -0700
Message-ID: <CA+55aFzbom=qFc2pYk07XhiMBn083EXugSUHmSVbTuu8eJtHVQ@mail.gmail.com>
Subject: Re: [PATCH 1/2] sched/wait: Break up long wake list walk
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: "Liang, Kan" <kan.liang@intel.com>,
        Mel Gorman <mgorman@techsingularity.net>, Mel Gorman <mgorman@suse.de>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
        Andi Kleen <ak@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>, Jan Kara <jack@suse.cz>,
        linux-mm <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Aug 23, 2017 at 8:58 AM, Tim Chen <tim.c.chen@linux.intel.com> wrote:
>
> Will you still consider the original patch as a fail safe mechanism?

I don't think we have much choice, although I would *really* want to
get this root-caused rather than just papering over the symptoms.

Maybe still worth testing that "sched/numa: Scale scan period with
tasks in group and shared/private" patch that Mel mentioned.

In fact, looking at that patch description, it does seem to match this
particular load a lot. Quoting from the commit message:

  "Running 80 tasks in the same group, or as threads of the same process,
   results in the memory getting scanned 80x as fast as it would be if a
   single task was using the memory.

   This really hurts some workloads"

So if 80 threads causes 80x as much scanning, a few thousand threads
might indeed be really really bad.

So once more unto the breach, dear friends, once more.

Please.

The patch got applied to -tip as commit b5dd77c8bdad, and can be
downloaded here:

  https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=b5dd77c8bdada7b6262d0cba02a6ed525bf4e6e1

(Hmm. It says it's cc'd to me, but I never noticed that patch simply
because it was in a big group of other -tip commits.. Oh well).

          Linus