linux-kernel - Re: [RFC 8/8] Do not reclaim the whole CPU bandwidth

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Message-ID: <5698ABFD.1040704@unitn.it>
Date:	Fri, 15 Jan 2016 09:21:17 +0100
From:	Luca Abeni <luca.abeni@...tn.it>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@....com>
Subject: Re: [RFC 8/8] Do not reclaim the whole CPU bandwidth

On 01/14/2016 08:59 PM, Peter Zijlstra wrote:
> On Thu, Jan 14, 2016 at 04:24:53PM +0100, Luca Abeni wrote:
>> Original GRUB tends to reclaim 100% of the CPU time... And this allows a
>> "CPU hog" (i.e., a busy loop) to starve non-deadline tasks.
>> To address this issue, allow the scheduler to reclaim only a specified
>> fraction of CPU time.
>> NOTE: the fraction of CPU time that cannot be reclaimed is currently
>> hardcoded as (1 << 20) / 10 -> 90%, but it must be made configurable!
>
> So the alternative is an explicit SCHED_OTHER server which is
> configurable.
Yes, I have thought about something similar (actually, this is the strategy
I implemented in my first CBS/GRUB scheduler. With the "old" 2.4 scheduler,
this was easier :).
But I think the solution I implemented in this patch is much simpler (it
just requires a very simple modification to grub_reclaim()) and is more
elegant from the theoretical point of view.


> That would maybe fit in nicely with the DL based FIFO/RR servers from
> this other pending project.
Yes, this reminds me about the half-finished patch for RT throttling using
SCHED_DEADLINE... But that patch needs much more work IMHO.


				Thanks,
					Luca
>
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -524,6 +524,10 @@ struct dl_rq {
>>   	 * and decreased when a task blocks
>>   	 */
>>   	s64 running_bw;
>> +	/* This is the amount of utilization that GRUB can not
>> +         * reclaim (per runqueue)
>> +         */
>> +	s64 unusable_bw;
>
>
> Wrong comment style and whitespace challenged.
>