Messages in this thread | | | Date | Mon, 15 Aug 2022 17:49:07 +0800 | Subject | Re: [PATCH v4 5/7] sched/fair: skip SIS domain search if fully busy | From | Abel Wu <> |
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Hi Gautham, thanks for your reviewing and sorry for my late reply..
On 7/20/22 11:34 PM, Gautham R. Shenoy Wrote: > > [..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 ? >
Makes sense, will rename in next version.
> >> { >> 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.
OK, I will make the param more verbose...
> >> +{ >> + 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)
and this one too.
>> +{ >> + 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
For now, yes. But sd_state can be expanded and once that happens, the positive check could be error prone.
> > 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.
Yes, indeed. We will talk about this later in the 7th patch.
> > Apart from this, patch looks good to me.
Thanks!
> > 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. >
Sounds like a good idea!
Best Regards, Abel
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