linux-kernel - Re: [PATCH Resend v6] sched: fix wrong rq's runnable

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: <CAKfTPtBuAwyiNYk17SnW066FhvGF18n+HaiKwhKLgsii8ntcHA@mail.gmail.com>
Date:	Fri, 19 Apr 2013 10:50:19 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Mike Galbraith <efault@....de>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Paul Turner <pjt@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH Resend v6] sched: fix wrong rq's runnable_avg update with
 rt tasks

On 19 April 2013 10:14, Mike Galbraith <efault@....de> wrote:
> On Fri, 2013-04-19 at 09:49 +0200, Vincent Guittot wrote:
>> On 19 April 2013 06:30, Mike Galbraith <efault@....de> wrote:
>> > On Thu, 2013-04-18 at 18:34 +0200, Vincent Guittot wrote:
>> >> The current update of the rq's load can be erroneous when RT tasks are
>> >> involved
>> >>
>> >> The update of the load of a rq that becomes idle, is done only if the avg_idle
>> >> is less than sysctl_sched_migration_cost. If RT tasks and short idle duration
>> >> alternate, the runnable_avg will not be updated correctly and the time will be
>> >> accounted as idle time when a CFS task wakes up.
>> >>
>> >> A new idle_enter function is called when the next task is the idle function
>> >> so the elapsed time will be accounted as run time in the load of the rq,
>> >> whatever the average idle time is. The function update_rq_runnable_avg is
>> >> removed from idle_balance.
>> >>
>> >> When a RT task is scheduled on an idle CPU, the update of the rq's load is
>> >> not done when the rq exit idle state because CFS's functions are not
>> >> called. Then, the idle_balance, which is called just before entering the
>> >> idle function, updates the rq's load and makes the assumption that the
>> >> elapsed time since the last update, was only running time.
>> >>
>> >> As a consequence, the rq's load of a CPU that only runs a periodic RT task,
>> >> is close to LOAD_AVG_MAX whatever the running duration of the RT task is.
>> >
>> > Why do we care what rq's load says, if the only thing running is a
>> > periodic RT task?  I _think_ I recall that stuff being put under the
>>
>> cfs scheduler will use a wrong rq load the next time it wants to schedule a task
>>
>> > throttle specifically to not waste cycles doing that on every
>> > microscopic idle.
>>
>> yes but this lead to the wrong computation of runnable_avg_sum. To be
>> more precise, we only need to call __update_entity_runnable_avg,
>> __update_tg_runnable_avg is not mandatory in this step.
>
> If it only scares fair class tasks away from the periodic rt load, that
> seems like a benefit to me, not a liability.  If we really really need

I'm not sure that such behavior that is only based on erroneous value,
is good one.

> perfect load numbers, fine, we have to eat some cycles, but when I look
> at it, it looks like one of those "Perfect is the enemy of good" things.

The target is not perfect number but good enough to be usable. The
systctl_migration_cost threshold is good for idle balancing but can
generates wrong load value

Vincent
>
> -Mike
>
--
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/