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

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Date:	Tue, 06 May 2014 16:20:59 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 05/06/2014 09:25 AM, Peter Zijlstra wrote:
> 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 :-(

It appears to be the case on the vast majority of the NUMA
systems that are actually in use.

To me, this suggests that we should probably deal with it.

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