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: <492406FD.3030001@novell.com>
Date:	Wed, 19 Nov 2008 07:30:53 -0500
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Max Krasnyansky <maxk@...lcomm.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]

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

-Greg


>   
>> I guess it can all be rather
>> transparently masked via a userspace tool, but we don't have a common
>> one yet.
>>     
> I do :). It's called 'syspart'
> 	http://git.kernel.org/?p=linux/kernel/git/maxk/syspart.git;a=summary
> I'll push an updated version in a couple of days.
>
>   
>> I do have a question, though: is your recommendation to just turn the
>> load balancer off in the cpuset you create that has the isolated CPUs?
>> I guess the conceptual issue I was having was that the root cpuset (I
>> think) always contains all CPUs and all memory nodes. So even if you
>> put some CPUs in a cpuset under the root one, and isolate them using
>> hotplug + disabling the load balancer in that cpuset, those CPUs are
>> still available to tasks in the root cpuset? Maybe I'm just missing a
>> step in the configuration, but it seems like as long as the global
>> (root cpuset) load balancer is on, a CPU can't be guaranteed to stay
>> isolated?
>>     
> Take a look at what 'syspart' does. In short yes, of course we need to set
> sched_load_balance flag in root cpuset to 0.
>
> Max
>
>
>
>
>
>   



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