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    SubjectRe: [PATCH v7 2/7] sched: move cfs task on a CPU with higher capacity
    On 9 October 2014 17:30, Peter Zijlstra <peterz@infradead.org> wrote:
    > On Thu, Oct 09, 2014 at 04:59:36PM +0200, Vincent Guittot wrote:
    >> On 9 October 2014 13:23, Peter Zijlstra <peterz@infradead.org> wrote:
    >> > On Tue, Oct 07, 2014 at 02:13:32PM +0200, Vincent Guittot wrote:
    >> >> +++ b/kernel/sched/fair.c
    >> >> @@ -5896,6 +5896,18 @@ fix_small_capacity(struct sched_domain *sd, struct sched_group *group)
    >> >> }
    >> >>
    >> >> /*
    >> >> + * Check whether the capacity of the rq has been noticeably reduced by side
    >> >> + * activity. The imbalance_pct is used for the threshold.
    >> >> + * Return true is the capacity is reduced
    >> >> + */
    >> >> +static inline int
    >> >> +check_cpu_capacity(struct rq *rq, struct sched_domain *sd)
    >> >> +{
    >> >> + return ((rq->cpu_capacity * sd->imbalance_pct) <
    >> >> + (rq->cpu_capacity_orig * 100));
    >> >> +}
    >> >> +
    >> >> +/*
    >> >> * Group imbalance indicates (and tries to solve) the problem where balancing
    >> >> * groups is inadequate due to tsk_cpus_allowed() constraints.
    >> >> *
    >> >> @@ -6567,6 +6579,14 @@ static int need_active_balance(struct lb_env *env)
    >> >> */
    >> >> if ((sd->flags & SD_ASYM_PACKING) && env->src_cpu > env->dst_cpu)
    >> >> return 1;
    >> >> +
    >> >> + /*
    >> >> + * The src_cpu's capacity is reduced because of other
    >> >> + * sched_class or IRQs, we trig an active balance to move the
    >> >> + * task
    >> >> + */
    >> >> + if (check_cpu_capacity(env->src_rq, sd))
    >> >> + return 1;
    >> >> }
    >> >
    >> > So does it make sense to first check if there's a better candidate at
    >> > all? By this time we've already iterated the current SD while trying
    >> > regular load balancing, so we could know this.
    >>
    >> i'm not sure to completely catch your point.
    >> Normally, f_b_g and f_b_q have already looked at the best candidate
    >> when we call need_active_balance. And src_cpu has been elected.
    >> Or i have missed your point ?
    >
    > Yep you did indeed miss my point.
    >
    > So I've always disliked this patch for its arbitrary nature, why
    > unconditionally try and active balance every time there is 'some' RT/IRQ
    > usage, it could be all CPUs are over that arbitrary threshold and we'll
    > end up active balancing for no point.
    >
    > So, since we've already iterated all CPUs in our domain back in
    > update_sd_lb_stats() we could have computed the CFS fraction:
    >
    > 1024 * capacity / capacity_orig
    >
    > for every cpu and collected the min/max of this. Then we can compute if
    > src is significantly (and there I suppose we can indeed use imb)
    > affected compared to others.

    ok, so we should put additional check in f_b_g to be sure that we will
    jump to force_balance only if there will be a real gain by moving the
    task on the local group (from an available capacity for the task point
    of view)
    and probably in f_b_q too

    >
    >


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