linux-kernel - Re: Using cpusets for configuration/isolation [Was Re: RT sched: cpupri_vec lock contention with def_root

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Message-ID: <49243E9D.1050209@qualcomm.com>
Date:	Wed, 19 Nov 2008 08:28:13 -0800
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	Gregory Haskins <ghaskins@...ell.com>
CC:	Nish Aravamudan <nish.aravamudan@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dimitri Sivanich <sivanich@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Using cpusets for configuration/isolation [Was Re: RT sched:
 cpupri_vec lock contention with def_root_domain and no load balance]



Gregory Haskins wrote:
> Max Krasnyansky wrote:
>> Nish Aravamudan wrote:
>>   
>>> On Tue, Nov 18, 2008 at 5:59 PM, Max Krasnyansky <maxk@...lcomm.com> wrote:
>>>     
>>>> I do not see how 'partfs' that you described would be different from
>>>> 'cpusets' that we have now. Just ignore 'tasks' files in the cpusets and you
>>>> already have your 'partfs'. You do _not_ have to use cpuset for assigning
>>>> tasks if you do not want to. Just use them to define sets of cpus and keep
>>>> all the tasks in the 'root' set. You can then explicitly pin your threads
>>>> down with pthread_set_affinity().
>>>>       
>>> I guess you're right. It still feels a bit kludgy, but that is probably just me.
>>>
>>> I have wondered, though, if it makes sense to provide an "isolated"
>>> file in /sys/devices/system/cpu/cpuX/ to do most of the offline
>>> sequence, break sched_domains and remove a CPU from the load balancer
>>> (rather than turning the load balancer off), rather than requiring a
>>> user to explicitly do an offline/online. 
>>>     
>> I do not see any benefits in exposing a special 'isolated' bit and have it do
>> the same thing that the cpu hotplug already does. As I explained in other
>> threads cpu hotplug is a _perfect_ fit for the isolation purposes. In order to
>> isolate a CPU dynamically (ie at runtime) we need to flush pending work, flush
>> chaches, move tasks and timers, etc. Which is _exactly_ what cpu hotplug code
>> does when it brings CPU down. There is no point in reimplementing it.
>>
>> btw It sounds like you misunderstood the meaning of the
>> cpuset.sched_load_balance flag. It's does not turn really turn load balancer
>> off, it simply causes cpus in different cpusets to be put into separate sched
>> domains. In other words it already does exactly what you're asking for.
>>   
> 
> On a related note, please be advised I have a bug in this area:
> 
> http://bugzilla.kernel.org/show_bug.cgi?id=12054

Yes, I saw the original thread on this. I'll reply in there.

Max
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