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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 21 Dec 2016 10:02:27 -0800
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Subject: Re: [RFC][PATCH] make global bitlock waitqueues per-node
To: Peter Zijlstra <peterz@infradead.org>, Nick Piggin <npiggin@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
        Bob Peterson <rpeterso@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        linux-mm <linux-mm@kvack.org>
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On Wed, Dec 21, 2016 at 12:32 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> FWIW, here's mine.. compiles and boots on a NUMA x86_64 machine.

So I like how your patch is smaller, but your patch is also broken.

First off, the whole contention bit is *not* NUMA-specific. It should
help non-NUMA too, by avoiding the stupid extra cache miss.

Secondly, CONFIG_NUMA is a broken thing to test anyway, since adding a
bit for the NUMA case can overflow the page flags as far as I can tell
(MIPS seems to support NUMA on 32-bit, for example, but I didn't
really check the Kconfig details). Making it dependent on 64-bit might
be ok (and would fix the issue above - I don't think we really need to
care too much about 32-bit any more)

But making it conditional at all means that now you have those two
different cases for this, which is a maintenance nightmare. So don't
do it even if we could say "screw 32-bit".

Anyway, the conditional thing could be fixed by just taking Nick's
patch 1/2, and your patch (with the conditional bits stripped out).

I do think your approach of just re-using the existing bit waiting
with just a page-specific waiting function is nicer than Nick's "let's
just roll new waiting functions" approach. It also avoids the extra
initcall.

Nick, comments?

Hugh - mind testing PeterZ's patch too? My comments about the
conditional PG_waiters bit and page bit overflow are not relevant for
your particular scenario, so you can ignore that part, and just take
PaterZ's patch directly.

               Linus