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Date: Tue, 22 Aug 2017 21:08:28 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 1/2] sched/wait: Break up long wake list walk
Message-ID: <20170822190828.GO32112@worktop.programming.kicks-ass.net>
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To: Linus Torvalds <torvalds@linux-foundation.org>
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>, Tim Chen <tim.c.chen@linux.intel.com>, 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>

On Tue, Aug 22, 2017 at 11:19:12AM -0700, Linus Torvalds wrote:
> And since everybody touches it, as a result everybody eventually
> thinks that page should be migrated to their NUMA node.

So that migration stuff has a filter on, we need two consecutive numa
faults from the same page_cpupid 'hash', see
should_numa_migrate_memory().

And since this appears to be anonymous memory (no THP) this is all a
single address space. However, we don't appear to invalidate TLBs when
we upgrade the PTE protection bits (not strictly required of course), so
we can have multiple CPUs trip over the same 'old' NUMA PTE.

Still, generating such a migration storm would be fairly tricky I think.

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