linux-kernel - Re: [PATCH RFC/TEST] sched: make sync affine wakeups work

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Message-ID: <20140506132516.GJ11096@twins.programming.kicks-ass.net>
Date:	Tue, 6 May 2014 15:25:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Preeti Murthy <preeti.lkml@...il.com>, umgwanakikbuti@...il.com,
	LKML <linux-kernel@...r.kernel.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Ingo Molnar <mingo@...nel.org>, george.mccollister@...il.com,
	ktkhai@...allels.com, Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Subject: Re: [PATCH RFC/TEST] sched: make sync affine wakeups work

On Sun, May 04, 2014 at 08:41:09AM -0400, Rik van Riel wrote:
> Even on 8-node DL980 systems, the NUMA distance in the
> SLIT table is less than RECLAIM_DISTANCE, and we will
> do wake_affine across the entire system.

Yeah, so the problem is that (AFAIK) ACPI doesn't actually specify a
metric for the SLIT distance. This (in as far as BIOS people would care
to stick to specs anyhow) has lead to the 'fun' situation where BIOS
engineers tweak SLIT table values to make OSes behave as they thing it
should.

So if the BIOS engineer finds that this system should have <
RECLAIM_DISTANCE it will simply make the table such that the max SLIT
value is below that.

And yes, I've seen this :-(

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