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

[Dietmar Eggemann]

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@xxxxxxxxxx>
> ---
>   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.

> +	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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