linux-kernel - Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt

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]

Date:	Mon, 14 Dec 2015 16:56:17 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>,
	Luca Abeni <luca.abeni@...tn.it>
Subject: Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity

On 14 December 2015 at 16:17, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Dec 08, 2015 at 10:19:30PM -0800, Steve Muckle wrote:
>> From: Vincent Guittot <vincent.guittot@...aro.org>
>
>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>> index 8b0a15e..9d9eb50 100644
>> --- a/kernel/sched/deadline.c
>> +++ b/kernel/sched/deadline.c
>> @@ -43,6 +43,24 @@ static inline int on_dl_rq(struct sched_dl_entity *dl_se)
>>       return !RB_EMPTY_NODE(&dl_se->rb_node);
>>  }
>>
>> +static void add_average_bw(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
>> +{
>> +     u64 se_bw = dl_se->dl_bw;
>> +
>> +     dl_rq->avg_bw += se_bw;
>> +}
>> +
>> +static void clear_average_bw(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
>> +{
>> +     u64 se_bw = dl_se->dl_bw;
>> +
>> +     dl_rq->avg_bw -= se_bw;
>> +     if (dl_rq->avg_bw < 0) {
>> +             WARN_ON(1);
>> +             dl_rq->avg_bw = 0;
>> +     }
>> +}
>
>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 4c49f76..ce05f61 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6203,6 +6203,14 @@ static unsigned long scale_rt_capacity(int cpu)
>>
>>       used = div_u64(avg, total);
>>
>> +     /*
>> +      * deadline bandwidth is defined at system level so we must
>> +      * weight this bandwidth with the max capacity of the system.
>> +      * As a reminder, avg_bw is 20bits width and
>> +      * scale_cpu_capacity is 10 bits width
>> +      */
>> +     used += div_u64(rq->dl.avg_bw, arch_scale_cpu_capacity(NULL, cpu));
>> +
>>       if (likely(used < SCHED_CAPACITY_SCALE))
>>               return SCHED_CAPACITY_SCALE - used;
>>
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 08858d1..e44c6be 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -519,6 +519,8 @@ struct dl_rq {
>>  #else
>>       struct dl_bw dl_bw;
>>  #endif
>> +     /* This is the "average utilization" for this runqueue */
>> +     s64 avg_bw;
>>  };
>
> So I don't think this is right. AFAICT this projects the WCET as the
> amount of time actually used by DL. This will, under many circumstances,
> vastly overestimate the amount of time actually spend on it. Therefore
> unduly pessimisme the fair capacity of this CPU.

I agree that if the WCET is far from reality, we will underestimate
available capacity for CFS. Have you got some use case in mind which
overestimates the WCET ?
If we can't rely on this parameters to evaluate the amount of capacity
used by deadline scheduler on a core, this will imply that we can't
also use it for requesting capacity to cpufreq and we should fallback
on a monitoring mechanism which reacts to a change instead of
anticipating it.

>
>
--
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/