linux-kernel - Re: [PATCH 03/14] sched: pack small tasks

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAKfTPtBTs17yc_Ff9Es2m6kmvsqidwFjAxYbR0sMJSFBeGGA6A@mail.gmail.com>
Date:	Fri, 26 Apr 2013 15:16:33 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Ingo Molnar <mingo@...nel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Paul Turner <pjt@...gle.com>,
	Santosh <santosh.shilimkar@...com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Chander Kashyap <chander.kashyap@...aro.org>,
	"cmetcalf@...era.com" <cmetcalf@...era.com>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Len Brown <len.brown@...el.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Jonathan Corbet <corbet@....net>,
	Lukasz Majewski <l.majewski@...sung.com>
Subject: Re: [PATCH 03/14] sched: pack small tasks

On 26 April 2013 14:30, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Apr 25, 2013 at 07:23:19PM +0200, Vincent Guittot wrote:
>> During the creation of sched_domain, we define a pack buddy CPU for each CPU
>> when one is available. We want to pack at all levels where a group of CPUs can
>> be power gated independently from others.
>> On a system that can't power gate a group of CPUs independently, the flag is
>> set at all sched_domain level and the buddy is set to -1. This is the default
>> behavior.
>>
>> On a dual clusters / dual cores system which can power gate each core and
>> cluster independently, the buddy configuration will be :
>>
>>       | Cluster 0   | Cluster 1   |
>>       | CPU0 | CPU1 | CPU2 | CPU3 |
>> -----------------------------------
>> buddy | CPU0 | CPU0 | CPU0 | CPU2 |
>>
>> If the cores in a cluster can't be power gated independently, the buddy
>> configuration becomes:
>>
>>       | Cluster 0   | Cluster 1   |
>>       | CPU0 | CPU1 | CPU2 | CPU3 |
>> -----------------------------------
>> buddy | CPU0 | CPU1 | CPU0 | CPU0 |
>>
>> Small tasks tend to slip out of the periodic load balance so the best place
>> to choose to migrate them is during their wake up. The decision is in O(1) as
>> we only check again one buddy CPU
>
>
> So I really don't get the point of this buddy stuff, even for light load non
> performance impact stuff you want to do.
>
> The moment you judge cpu0 busy you'll bail, even though its perfectly doable
> (and desirable afaict) to continue stacking light tasks on cpu1 instead of
> waking up cpu2/3.
>
> So what's wrong with keeping a single light-wake target cpu selection and
> updating it appropriately?

I have tried to follow the same kind of tasks migration as during load
balance: 1 CPU in a power domain group migrates tasks with other
groups.

>
> Also where/how does the nohz balance cpu criteria not match the light-wake
> target criteria?

The nohz balance cpu is an idle cpu but it doesn't mean that it's the
target cpu which will be sometime busy with the light tasks.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/