linux-kernel - Re: [PATCH] cpu hotplug, sched: Introduce cpu_active_map and redoscheddomain managment (take 2)

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Date:	Thu, 17 Jul 2008 11:52:57 -0700
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	Gregory Haskins <ghaskins@...ell.com>
CC:	a.p.zijlstra@...llo.nl, mingo@...e.hu, dmitry.adamushko@...il.com,
	torvalds@...ux-foundation.org, pj@....com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpu hotplug, sched: Introduce	cpu_active_map	and	redoscheddomain
 managment (take 2)

Gregory Haskins wrote:
>>>> On Thu, Jul 17, 2008 at  3:16 AM, in message <487EF1E9.2040101@...lcomm.com>,
> Max Krasnyansky <maxk@...lcomm.com> wrote: 
> 
>> Gregory Haskins wrote:
>>> Well, admittedly I am not entirely clear on what problem is being solved as
>>> I was not part of the original thread with Linus.  My impression of what you
>>> were trying to solve was to eliminate the need to rebuild the domains for a
>>> hotplug event (which I think is a good problem to solve), thus eliminating
>>> some complexity and (iiuc) races there.
>>>
>>> However, based on what you just said, I am not sure I've got that entirely
>>> right anymore.  Can you clarify the intent (or point me at the original 
>> thread)
>>> so we are on the same page?
>> Here is the link to the original thread
>> 	http://lkml.org/lkml/2008/7/11/328
>> And here is where Linus explained the idea
>> 	http://lkml.org/lkml/2008/7/12/137
>>
>> I'll reply to the rest of your email tomorrow (can't keep my yes open any
>> longer :)).
>>
>> Max
> 
> Hi Max,
>   Thanks for the pointers.  I see that I did indeed misunderstand the intent of the patch.
> It seems you already solved the rebuild problem, and were just trying to solve the
> "migrate to a dead cpu" problem that Linus mentions as a solution with cpu_active_map.
Yes. btw they are definitely related, because the reason we were blowing away
the domains is to avoid "migration to a dead cpu". ie We were relying on the
fact that domain masks never contain cpus that are either dying or already dead.

> In that case, note that rq->rd->online already fits the bill, I believe.  In a nutshell,
> rq->rd->span contains all the cpus within your disjoint cpuset, and rq->rd->online,
> contains the subset of rq->rd->span that are online.  The online bit is cleared at the
> earliest point in cpu hotplug removal (DYING), and it is set at the very latest point on
> insertion (ONLINE).  Therefore it is redundant with the cpus_active_map concept.
> 
> I think the simplest solution is to make sure that we cpus_and against rq->rd->online
> before allowing a migration.  This is how I intended the mask to be used, anyway.  Its
> what the RT scheduler does.  It sounds like we just need to touch up the few places
> in the CFS side that were causing those oops.
> 
> Thoughts?
None at this point :). I need to run right now and will try to look at this
later today. My knowledge of the internal sched structs is definitely lacking
so I need to look at the rq->rd thing to have and opinion.

Thanx
Max


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