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Message-ID: <c78b1dab-95fb-ed88-30de-b4d489a5fc17@oracle.com>
Date: Wed, 31 Jan 2018 09:50:56 -0800
From: Rohit Jain <rohit.k.jain@...cle.com>
To: Joel Fernandes <joelaf@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
LKML <linux-kernel@...r.kernel.org>, steven.sistare@...cle.com,
dhaval.giani@...cle.com,
Dietmar Eggemann <dietmar.eggemann@....com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Morten Rasmussen <morten.rasmussen@....com>,
"Cc: EAS Dev" <eas-dev@...ts.linaro.org>
Subject: Re: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in
select_idle_sibling
On 01/30/2018 05:57 PM, Joel Fernandes wrote:
<snip>
>>>> Signed-off-by: Rohit Jain<rohit.k.jain@...cle.com>
>>>> ---
>>>> kernel/sched/fair.c | 38 ++++++++++++++++++++++++++++----------
>>>> 1 file changed, 28 insertions(+), 10 deletions(-)
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index 26a71eb..ce5ccf8 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -5625,6 +5625,11 @@ static unsigned long capacity_orig_of(int cpu)
>>>> return cpu_rq(cpu)->cpu_capacity_orig;
>>>> }
>>>>
>>>> +static inline bool full_capacity(int cpu)
>>>> +{
>>>> + return capacity_of(cpu) >= (capacity_orig_of(cpu)*3)/4;
>>>> +}
>>>> +
>>>> static unsigned long cpu_avg_load_per_task(int cpu)
>>>> {
>>>> struct rq *rq = cpu_rq(cpu);
>>>> @@ -6081,7 +6086,7 @@ static int select_idle_core(struct task_struct *p,
>>>> struct sched_domain *sd, int
>>>>
>>>> for_each_cpu(cpu, cpu_smt_mask(core)) {
>>>> cpumask_clear_cpu(cpu, cpus);
>>>> - if (!idle_cpu(cpu))
>>>> + if (!idle_cpu(cpu) || !full_capacity(cpu))
>>>> idle = false;
>>>> }
>>> There's some difference in logic between select_idle_core and
>>> select_idle_cpu as far as the full_capacity stuff you're adding goes.
>>> In select_idle_core, if all CPUs are !full_capacity, you're returning
>>> -1. But in select_idle_cpu you're returning the best idle CPU that's
>>> the most cap among the !full_capacity ones. Why there is this
>>> different in logic? Did I miss something?
>>>
<snip>
> Dude :) That is hardly an answer to the question I asked. Hint:
> *different in logic*.
Let me re-try :)
For select_idle_core, we are doing a search for a fully idle and full
capacity core, the fail-safe is select_idle_cpu because we will re-scan
the CPUs. The notion is to select an idle CPU no matter what, because
being on an idle CPU is better than waiting on a non-idle one. In
select_idle_core you can be slightly picky about the core because
select_idle_cpu is a fail safe. I measured the performance impact of
choosing the "best among low cap" vs the code changes I have (for
select_idle_core) and could not find a statistically significant impact,
hence went with the simpler code changes.
Hope I answered your question.
Thanks,
Rohit
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