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Message-ID: <YtggcHGKVSbprXr3@BLR-5CG11610CF.amd.com>
Date: Wed, 20 Jul 2022 21:04:08 +0530
From: "Gautham R. Shenoy" <gautham.shenoy@....com>
To: Abel Wu <wuyun.abel@...edance.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Mel Gorman <mgorman@...e.de>,
Vincent Guittot <vincent.guittot@...aro.org>,
Josh Don <joshdon@...gle.com>, Chen Yu <yu.c.chen@...el.com>,
Tim Chen <tim.c.chen@...ux.intel.com>,
K Prateek Nayak <kprateek.nayak@....com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/7] sched/fair: skip SIS domain search if fully busy
Hello Abel,
On Sun, Jun 19, 2022 at 08:04:49PM +0800, Abel Wu wrote:
> If a full scan on SIS domain failed, then no unoccupied cpus available
> and the LLC is fully busy. In this case we'd better spend the time on
> something more useful, rather than wasting it trying to find an idle
> cpu that probably not exist.
>
> The fully busy status will be re-evaluated when any core of this LLC
> domain enters load balancing, and cleared once idle cpus found.
>
> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
> ---
[..snip..]
> @@ -6197,24 +6201,44 @@ static inline int __select_idle_cpu(int cpu, struct task_struct *p)
> DEFINE_STATIC_KEY_FALSE(sched_smt_present);
> EXPORT_SYMBOL_GPL(sched_smt_present);
>
> -static inline void set_idle_cores(int cpu, int val)
> +static inline void sd_set_state(int cpu, enum sd_state state)
Nit: We are setting the state of only the LLC domain and not any other
domain via this function. So should we name it as
set_llc_state()/get_llc_state() for better readability ?
> {
> struct sched_domain_shared *sds;
>
> sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> if (sds)
> - WRITE_ONCE(sds->has_idle_cores, val);
> + WRITE_ONCE(sds->state, state);
> }
>
> -static inline bool test_idle_cores(int cpu)
> +static inline enum sd_state sd_get_state(int cpu)
> {
> struct sched_domain_shared *sds;
>
> sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> if (sds)
> - return READ_ONCE(sds->has_idle_cores);
> + return READ_ONCE(sds->state);
>
> - return false;
> + return sd_has_icpus;
> +}
> +
> +static inline void set_idle_cores(int cpu, int idle)
^^^^^
I agree with Josh. We can use core_idle instead of idle here.
> +{
> + sd_set_state(cpu, idle ? sd_has_icores : sd_has_icpus);
> +}
> +
> +static inline bool test_idle_cores(int cpu)
> +{
> + return sd_get_state(cpu) == sd_has_icores;
> +}
> +
> +static inline void set_idle_cpus(int cpu, int idle)
> +{
> + sd_set_state(cpu, idle ? sd_has_icpus : sd_is_busy);
> +}
> +
> +static inline bool test_idle_cpus(int cpu)
> +{
> + return sd_get_state(cpu) != sd_is_busy;
> }
>
> /*
[...]
> @@ -8661,6 +8702,12 @@ sched_asym(struct lb_env *env, struct sd_lb_stats *sds, struct sg_lb_stats *sgs
> return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu);
> }
>
> +static inline void sd_classify(struct sd_lb_stats *sds, struct rq *rq)
> +{
> + if (sds->sd_state != sd_has_icpus && unoccupied_rq(rq))
Nit: sds->sd_state can either be sd_has_icpus or sd_is_busy. So for
better readability, we can just use the positive check
if (sds->sd_state == sd_is_busy && unoccupied_rq(rq))
sds->sd_state = sd_has_icpus;
> + sds->sd_state = sd_has_icpus;
> +}
> +
> /**
> * update_sg_lb_stats - Update sched_group's statistics for load balancing.
> * @env: The load balancing environment.
> @@ -8675,11 +8722,12 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> struct sg_lb_stats *sgs,
> int *sg_status)
> {
> - int i, nr_running, local_group;
> + int i, nr_running, local_group, update_core;
>
> memset(sgs, 0, sizeof(*sgs));
>
> local_group = group == sds->local;
> + update_core = env->sd->flags & SD_SHARE_CPUCAPACITY;
>
> for_each_cpu_and(i, sched_group_span(group), env->cpus) {
> struct rq *rq = cpu_rq(i);
> @@ -8692,6 +8740,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> nr_running = rq->nr_running;
> sgs->sum_nr_running += nr_running;
>
> + if (update_core)
> + sd_classify(sds, rq);
> +
> if (nr_running > 1)
> *sg_status |= SG_OVERLOAD;
>
> @@ -9220,6 +9271,12 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
> return idlest;
> }
>
> +static void sd_update_state(struct lb_env *env, struct sd_lb_stats *sds)
> +{
> + if (sds->sd_state == sd_has_icpus && !test_idle_cpus(env->dst_cpu))
> + set_idle_cpus(env->dst_cpu, true);
We could enter this if condition when env->dst_cpu is the only idle
CPU in the SMT domain (which is likely to be the case every time we do
a NEW_IDLE balance). By the end of this load-balancing round, the
env->dst_cpu can pull a task from some other CPU and thereby no longer
remain idle but the LLC state would still be sd_has_icpus.
That would mean that some CPU on this LLC would do a full scan during
the wakeup only to find no idle CPU and reset the state to
sd_is_busy. Have you seen instances where this false-positive pattern
can result in wasteful scan thereby cause a performance degradation?
Ideally it should not be worse that what we currently have.
Apart from this, patch looks good to me.
I would be worth the while to explore if the LLC state can be used
early on in select_task_rq_fair() to determine if we need to do a
wake-affine or allow the task to stick to its previous LLC depending
on which among the previous LLC and the waker's LLC have an idle CPU.
--
Thanks and Regards
gautham.
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