linux-kernel - Re: [PATCH v10 11/11] sched: move cfs task on a CPU with higher capacity

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Date:	Thu, 26 Mar 2015 16:43:19 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
Cc:	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	"kamalesh@...ux.vnet.ibm.com" <kamalesh@...ux.vnet.ibm.com>,
	"riel@...hat.com" <riel@...hat.com>,
	"efault@....de" <efault@....de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH v10 11/11] sched: move cfs task on a CPU with higher capacity

On 26 March 2015 at 15:19, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> On 27/02/15 15:54, Vincent Guittot wrote:
>>
>> When a CPU is used to handle a lot of IRQs or some RT tasks, the remaining
>> capacity for CFS tasks can be significantly reduced. Once we detect such
>> situation by comparing cpu_capacity_orig and cpu_capacity, we trig an idle
>> load balance to check if it's worth moving its tasks on an idle CPU.
>> It's worth trying to move the task before the CPU is fully utilized to
>> minimize the preemption by irq or RT tasks.
>>
>> Once the idle load_balance has selected the busiest CPU, it will look for
>> an
>> active load balance for only two cases :
>> - there is only 1 task on the busiest CPU.
>> - we haven't been able to move a task of the busiest rq.
>>
>> A CPU with a reduced capacity is included in the 1st case, and it's worth
>> to
>> actively migrate its task if the idle CPU has got more available capacity
>> for
>> CFS tasks. This test has been added in need_active_balance.
>>
>> As a sidenote, this will not generate more spurious ilb because we already
>> trig an ilb if there is more than 1 busy cpu. If this cpu is the only one
>> that
>> has a task, we will trig the ilb once for migrating the task.
>>
>> The nohz_kick_needed function has been cleaned up a bit while adding the
>> new
>> test
>>
>> env.src_cpu and env.src_rq must be set unconditionnally because they are
>> used
>> in need_active_balance which is called even if busiest->nr_running equals
>> 1
>>
>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>> ---
>>   kernel/sched/fair.c | 69
>> ++++++++++++++++++++++++++++++++++++-----------------
>>   1 file changed, 47 insertions(+), 22 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 7420d21..e70c315 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6855,6 +6855,19 @@ static int need_active_balance(struct lb_env *env)
>>                         return 1;
>>         }
>>
>> +       /*
>> +        * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
>> +        * It's worth migrating the task if the src_cpu's capacity is
>> reduced
>> +        * because of other sched_class or IRQs if more capacity stays
>> +        * available on dst_cpu.
>> +        */
>> +       if ((env->idle != CPU_NOT_IDLE) &&
>> +           (env->src_rq->cfs.h_nr_running == 1)) {
>> +               if ((check_cpu_capacity(env->src_rq, sd)) &&
>> +                   (capacity_of(env->src_cpu)*sd->imbalance_pct <
>> capacity_of(env->dst_cpu)*100))
>> +                       return 1;
>> +       }
>> +
>>         return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2);
>>   }
>>
>> @@ -6954,6 +6967,9 @@ static int load_balance(int this_cpu, struct rq
>> *this_rq,
>>
>>         schedstat_add(sd, lb_imbalance[idle], env.imbalance);
>>
>> +       env.src_cpu = busiest->cpu;
>
>
> Isn't this 'env.src_cpu = busiest->cpu;' or 'env.src_cpu = cpu_of(busiest);'
> already needed due to the existing ASYM_PACKING check in
> need_active_balance() 'if ( ... && env->src_cpu > env->dst_cpu)' for
> CPU_NEWLY_IDLE? Otherwise like you said, in these 'busiest->nr_running
> equals 1' instances, env->src_cpu is un-initialized.

yes, i sent a fix for that purpose some times ago :
https://lkml.org/lkml/2013/2/12/158
but it has not gone further than the mailing list.

AFAICT, SD_ASYM_PACKING can't trig an active load balance on the cpu
with the lowest id without this fix as src_cpu is initialized to 0
which implies that 'env->src_cpu > env->dst_cpu' is always false

Vincent

>
>> +       env.src_rq = busiest;
>> +
>>         ld_moved = 0;
>>         if (busiest->nr_running > 1) {
>>                 /*
>> @@ -6963,8 +6979,6 @@ static int load_balance(int this_cpu, struct rq
>> *this_rq,
>>                  * correctly treated as an imbalance.
>>                  */
>>                 env.flags |= LBF_ALL_PINNED;
>> -               env.src_cpu   = busiest->cpu;
>> -               env.src_rq    = busiest;
>>                 env.loop_max  = min(sysctl_sched_nr_migrate,
>> busiest->nr_running);
>>
>>   more_balance:
>
>
> [...]
>
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