linux-kernel - Re: [PATCH] sched/rt: don't try to balance rt

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Message-ID: <5476EC32.9030005@gmail.com>
Date:	Thu, 27 Nov 2014 17:17:38 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/rt: don't try to balance rt_runtime when it is
 futile

Hi Mike,
On 5/15/14, 10:49 AM, Mike Galbraith wrote:
> On Wed, 2014-05-14 at 15:11 -0400, Paul Gortmaker wrote:
>
>> Given that, perhaps a separate change to sched_rt_runtime_exceeded()
>> that works out the CPU from the rt_rq, and returns zero if it is a
>> nohz_full cpu?  Does that make sense?  Then the nohz_full people won't
>> get the throttling message even if they go 100%.
> I don't get it.  What reason would there be to run a hog on a dedicated
> core as realtime policy/priority?  Given no competition, there's nothing
> to prioritize, you could just as well run a critical task as SCHED_IDLE.
>
> I would also expect that anyone wanting bare metal will have all of
> their critical cores isolated from the scheduler, watchdogs turned off
> as well as that noisy throttle, the whole point being to make as much
> silent as possible.  Seems to me tick_nohz_full_cpu(cpu) should be
> predicated by that cpu being isolated from the #1 noise source, the
> scheduler and its load balancing.  There's just no point to nohz_full
> without that, or if there is, I sure don't see it.

If the tick is still need to be handled if cpu is isolated w/o nohz full 
enabled?

Regards,
Wanpeng Li

>
> When I see people trying to run a hog as a realtime task, it's because
> they are trying in vain to keep competition away from precious cores..
> and one mlockall with a realtime hog blocking flush_work() gives them a
> wakeup call.
>
> -Mike
>
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