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Message-ID: <CAKfTPtCHhxyugMOCeS0hZoEXWhgRec4uOBexZ2aB2A+Eycgi8w@mail.gmail.com>
Date: Thu, 29 May 2014 21:37:39 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Russell King - ARM Linux <linux@....linux.org.uk>,
LAK <linux-arm-kernel@...ts.infradead.org>,
Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
Morten Rasmussen <Morten.Rasmussen@....com>,
Mike Galbraith <efault@....de>,
Nicolas Pitre <nicolas.pitre@...aro.org>,
"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v2 10/11] sched: move cfs task on a CPU with higher capacity
On 29 May 2014 11:50, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, May 23, 2014 at 05:53:04PM +0200, Vincent Guittot wrote:
>> If the CPU is used for handling lot of IRQs, trig a load balance to check if
>> it's worth moving its tasks on another CPU that has more capacity
>>
>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>> ---
>> kernel/sched/fair.c | 13 +++++++++++++
>> 1 file changed, 13 insertions(+)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index e8a30f9..2501e49 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -5948,6 +5948,13 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>> if (sgs->sum_nr_running > sgs->group_capacity)
>> return true;
>>
>> + /*
>> + * The group capacity is reduced probably because of activity from other
>> + * sched class or interrupts which use part of the available capacity
>> + */
>> + if ((sg->sgp->power_orig * 100) > (sgs->group_power * env->sd->imbalance_pct))
>> + return true;
>> +
>> if (sgs->group_imb)
>> return true;
>>
>
> But we should already do this because the load numbers are scaled with
> the power/capacity figures. If one CPU gets significant less time to run
> fair tasks, its effective load would spike and it'd get to be selected
> here anyway.
>
> Or am I missing something?
The CPU could have been picked when the capacity becomes null (which
occurred when the cpu_power goes below half the default
SCHED_POWER_SCALE). And even after that, there were some conditions in
find_busiest_group that was bypassing this busiest group
>
>> @@ -7282,6 +7289,12 @@ static inline int nohz_kick_needed(struct rq *rq)
>>
>> if (nr_busy > 1)
>> goto need_kick_unlock;
>> +
>> + if ((rq->cfs.h_nr_running >= 1)
>> + && ((rq->cpu_power * sd->imbalance_pct) <
>> + (rq->cpu_power_orig * 100)))
>> + goto need_kick_unlock;
>> +
>> }
>>
>> sd = rcu_dereference(per_cpu(sd_asym, cpu));
>
> OK, so there you're kicking the idle balancer to try and get another CPU
> to pull some load? That makes sense I suppose.
and especially if we have idle CPUs and one task on the CPU with
reduced capacity
>
> That function is pretty horrible though; how about something like this
> first?
ok, i will integrate this modification in next version
>
> ---
> kernel/sched/fair.c | 29 +++++++++++++----------------
> 1 file changed, 13 insertions(+), 16 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c9617b73bcc0..47fb96e6fa83 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7215,15 +7215,16 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> * - For SD_ASYM_PACKING, if the lower numbered cpu's in the scheduler
> * domain span are idle.
> */
> -static inline int nohz_kick_needed(struct rq *rq)
> +static inline bool nohz_kick_needed(struct rq *rq)
> {
> unsigned long now = jiffies;
> struct sched_domain *sd;
> struct sched_group_power *sgp;
> int nr_busy, cpu = rq->cpu;
> + bool kick = false;
>
> if (unlikely(rq->idle_balance))
> - return 0;
> + return false;
>
> /*
> * We may be recently in ticked or tickless idle mode. At the first
> @@ -7237,38 +7238,34 @@ static inline int nohz_kick_needed(struct rq *rq)
> * balancing.
> */
> if (likely(!atomic_read(&nohz.nr_cpus)))
> - return 0;
> + return false;
>
> if (time_before(now, nohz.next_balance))
> - return 0;
> + return false;
>
> if (rq->nr_running >= 2)
> - goto need_kick;
> + return true;
>
> rcu_read_lock();
> sd = rcu_dereference(per_cpu(sd_busy, cpu));
> -
> if (sd) {
> sgp = sd->groups->sgp;
> nr_busy = atomic_read(&sgp->nr_busy_cpus);
>
> - if (nr_busy > 1)
> - goto need_kick_unlock;
> + if (nr_busy > 1) {
> + kick = true;
> + goto unlock;
> + }
> }
>
> sd = rcu_dereference(per_cpu(sd_asym, cpu));
> -
> if (sd && (cpumask_first_and(nohz.idle_cpus_mask,
> sched_domain_span(sd)) < cpu))
> - goto need_kick_unlock;
> -
> - rcu_read_unlock();
> - return 0;
> + kick = true;
>
> -need_kick_unlock:
> +unlock:
> rcu_read_unlock();
> -need_kick:
> - return 1;
> + return kick;
> }
> #else
> static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) { }
--
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