Date: Tue, 22 Aug 2017 12:37:14 -0700
From: Andi Kleen <ak@linux.intel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
        "Liang, Kan" <kan.liang@intel.com>,
        Mel Gorman <mgorman@techsingularity.net>, Mel Gorman <mgorman@suse.de>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, Ingo Molnar <mingo@elte.hu>,
        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>
Subject: Re: [PATCH 1/2] sched/wait: Break up long wake list walk
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> > Still, generating such a migration storm would be fairly tricky I think.
> 
> Well, Mel seems to have been unable to generate a load that reproduces
> the long page waitqueues. And I don't think we've had any other
> reports of this either.

It could be that it requires a fairly large system. On large systems
under load a lot of things take much longer, so what's a tiny window on Mel's
system may suddenly be very large, and with much more threads 
they have a higher chance of bad interactions anyways.

We only see it on 4S+ today. But systems are always getting larger,
so what's a large system today, will be a normal medium scale system
tomorrow.

BTW we also collected PT traces for the long hang cases, but it was
hard to find a consistent pattern in them.

-Andi