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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 4/7] xen: credit2: soft-affinity awareness in csched2_cpu_pick()
On 06/16/2017 03:14 PM, Dario Faggioli wrote:
> We want to find the runqueue with the least average load,
> and to do that, we scan through all the runqueues.
>
> It is, therefore, enough that, during such scan:
> - we identify the runqueue with the least load, among
> the ones that have pcpus that are part of the soft
> affinity of the vcpu we're calling pick on;
> - we identify the same, but for hard affinity.
>
> At this point, we can decide whether to go for the
> runqueue with the least load among the ones with some
> soft-affinity, or overall.
>
> Therefore, at the price of some code reshuffling, we
> can avoid the loop.
>
> (Also, kill a spurious ';' in the definition of MAX_LOAD.)
Wow, that is kind of hilarious now that managed to slip its way in there
by always being behind a ';' anyway. :-)
>
> Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
> Signed-off-by: Justin T. Weaver <jtweaver@xxxxxxxxxx>
Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> ---
> Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
> Cc: Anshul Makkar <anshul.makkar@xxxxxxxxxx>
> ---
> xen/common/sched_credit2.c | 117
> ++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 97 insertions(+), 20 deletions(-)
>
> diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
> index 54f6e21..fb97ff7 100644
> --- a/xen/common/sched_credit2.c
> +++ b/xen/common/sched_credit2.c
> @@ -1725,14 +1725,16 @@ csched2_context_saved(const struct scheduler *ops,
> struct vcpu *vc)
> vcpu_schedule_unlock_irq(lock, vc);
> }
>
> -#define MAX_LOAD (STIME_MAX);
> +#define MAX_LOAD (STIME_MAX)
> static int
> csched2_cpu_pick(const struct scheduler *ops, struct vcpu *vc)
> {
> struct csched2_private *prv = csched2_priv(ops);
> - int i, min_rqi = -1, new_cpu, cpu = vc->processor;
> + int i, min_rqi = -1, min_s_rqi = -1;
> + unsigned int new_cpu, cpu = vc->processor;
> struct csched2_vcpu *svc = csched2_vcpu(vc);
> - s_time_t min_avgload = MAX_LOAD;
> + s_time_t min_avgload = MAX_LOAD, min_s_avgload = MAX_LOAD;
> + bool has_soft;
>
> ASSERT(!cpumask_empty(&prv->active_queues));
>
> @@ -1781,17 +1783,35 @@ csched2_cpu_pick(const struct scheduler *ops, struct
> vcpu *vc)
> else if ( cpumask_intersects(cpumask_scratch_cpu(cpu),
> &svc->migrate_rqd->active) )
> {
> + /*
> + * If we've been asked to move to migrate_rqd, we should just do
> + * that, which we actually do by returning one cpu from that
> runq.
> + * There is no need to take care of soft affinity, as that will
> + * happen in runq_tickle().
> + */
> cpumask_and(cpumask_scratch_cpu(cpu), cpumask_scratch_cpu(cpu),
> &svc->migrate_rqd->active);
> new_cpu = cpumask_cycle(svc->migrate_rqd->pick_bias,
> cpumask_scratch_cpu(cpu));
> +
> svc->migrate_rqd->pick_bias = new_cpu;
> goto out_up;
> }
> /* Fall-through to normal cpu pick */
> }
>
> - /* Find the runqueue with the lowest average load. */
> + /*
> + * What we want is:
> + * - if we have soft affinity, the runqueue with the lowest average
> + * load, among the ones that contain cpus in our soft affinity; this
> + * represents the best runq on which we would want to run.
> + * - the runqueue with the lowest average load among the ones that
> + * contains cpus in our hard affinity; this represent the best runq
> + * on which we can run.
> + *
> + * Find both runqueues in one pass.
> + */
> + has_soft = has_soft_affinity(vc, vc->cpu_hard_affinity);
> for_each_cpu(i, &prv->active_queues)
> {
> struct csched2_runqueue_data *rqd;
> @@ -1800,31 +1820,51 @@ csched2_cpu_pick(const struct scheduler *ops, struct
> vcpu *vc)
> rqd = prv->rqd + i;
>
> /*
> - * If checking a different runqueue, grab the lock, check hard
> - * affinity, read the avg, and then release the lock.
> + * If none of the cpus of this runqueue is in svc's hard-affinity,
> + * skip the runqueue.
> + *
> + * Note that, in case svc's hard-affinity has changed, this is the
> + * first time when we see such change, so it is indeed possible
> + * that we end up skipping svc's current runqueue.
> + */
> + if ( !cpumask_intersects(cpumask_scratch_cpu(cpu), &rqd->active) )
> + continue;
> +
> + /*
> + * If checking a different runqueue, grab the lock, read the avg,
> + * and then release the lock.
> *
> * If on our own runqueue, don't grab or release the lock;
> * but subtract our own load from the runqueue load to simulate
> * impartiality.
> - *
> - * Note that, if svc's hard affinity has changed, this is the
> - * first time when we see such change, so it is indeed possible
> - * that none of the cpus in svc's current runqueue is in our
> - * (new) hard affinity!
> */
> if ( rqd == svc->rqd )
> {
> - if ( cpumask_intersects(cpumask_scratch_cpu(cpu), &rqd->active) )
> - rqd_avgload = max_t(s_time_t, rqd->b_avgload - svc->avgload,
> 0);
> + rqd_avgload = max_t(s_time_t, rqd->b_avgload - svc->avgload, 0);
> }
> else if ( spin_trylock(&rqd->lock) )
> {
> - if ( cpumask_intersects(cpumask_scratch_cpu(cpu), &rqd->active) )
> - rqd_avgload = rqd->b_avgload;
> -
> + rqd_avgload = rqd->b_avgload;
> spin_unlock(&rqd->lock);
> }
>
> + /*
> + * if svc has a soft-affinity, and some cpus of rqd are part of it,
> + * see if we need to update the "soft-affinity minimum".
> + */
> + if ( has_soft &&
> + rqd_avgload < min_s_avgload )
> + {
> + cpumask_t mask;
> +
> + cpumask_and(&mask, cpumask_scratch_cpu(cpu), &rqd->active);
> + if ( cpumask_intersects(&mask, svc->vcpu->cpu_soft_affinity) )
> + {
> + min_s_avgload = rqd_avgload;
> + min_s_rqi = i;
> + }
> + }
> + /* In any case, keep the "hard-affinity minimum" updated too. */
> if ( rqd_avgload < min_avgload )
> {
> min_avgload = rqd_avgload;
> @@ -1832,17 +1872,54 @@ csched2_cpu_pick(const struct scheduler *ops, struct
> vcpu *vc)
> }
> }
>
> - /* We didn't find anyone (most likely because of spinlock contention). */
> - if ( min_rqi == -1 )
> + if ( has_soft && min_s_rqi != -1 )
> + {
> + /*
> + * We have soft affinity, and we have a candidate runq, so go for it.
> + *
> + * Note that, to obtain the soft-affinity mask, we "just" put what we
> + * have in cpumask_scratch in && with vc->cpu_soft_affinity. This is
> + * ok because:
> + * - we know that vc->cpu_hard_affinity and vc->cpu_soft_affinity
> have
> + * a non-empty intersection (because has_soft is true);
> + * - we have vc->cpu_hard_affinity & cpupool_domain_cpumask() already
> + * in cpumask_scratch, we do save a lot doing like this.
> + *
> + * It's kind of like open coding affinity_balance_cpumask() but, in
> + * this specific case, calling that would mean a lot of (unnecessary)
> + * cpumask operations.
> + */
> + cpumask_and(cpumask_scratch_cpu(cpu), cpumask_scratch_cpu(cpu),
> + vc->cpu_soft_affinity);
> + cpumask_and(cpumask_scratch_cpu(cpu), cpumask_scratch_cpu(cpu),
> + &prv->rqd[min_s_rqi].active);
> + }
> + else if ( min_rqi != -1 )
> {
> + /*
> + * Either we don't have soft-affinity, or we do, but we did not find
> + * any suitable runq. But we did find one when considering hard
> + * affinity, so go for it.
> + *
> + * cpumask_scratch already has vc->cpu_hard_affinity &
> + * cpupool_domain_cpumask() in it, so it's enough that we filter
> + * with the cpus of the runq.
> + */
> + cpumask_and(cpumask_scratch_cpu(cpu), cpumask_scratch_cpu(cpu),
> + &prv->rqd[min_rqi].active);
> + }
> + else
> + {
> + /*
> + * We didn't find anyone at all (most likely because of spinlock
> + * contention).
> + */
> new_cpu = get_fallback_cpu(svc);
> min_rqi = c2r(ops, new_cpu);
> min_avgload = prv->rqd[min_rqi].b_avgload;
> goto out_up;
> }
>
> - cpumask_and(cpumask_scratch_cpu(cpu), cpumask_scratch_cpu(cpu),
> - &prv->rqd[min_rqi].active);
> new_cpu = cpumask_cycle(prv->rqd[min_rqi].pick_bias,
> cpumask_scratch_cpu(cpu));
> prv->rqd[min_rqi].pick_bias = new_cpu;
>
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