Messages in this thread |  | | Date | Sat, 22 May 2021 19:40:45 +0530 | From | Srikar Dronamraju <> | Subject | Re: [PATCH v3 2/8] sched/fair: Maintain the identity of idle-core |
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* Vincent Guittot <vincent.guittot@linaro.org> [2021-05-22 14:42:00]:
> On Fri, 21 May 2021 at 15:31, Srikar Dronamraju > <srikar@linux.vnet.ibm.com> wrote: > > > > * Vincent Guittot <vincent.guittot@linaro.org> [2021-05-21 14:36:15]: > > > > > On Thu, 13 May 2021 at 09:40, Srikar Dronamraju > > > <srikar@linux.vnet.ibm.com> wrote: > > > > > > > > Scheduler maintains a per LLC info which tells if there is any idle core > > > > in that LLC. However this information doesn't provide which core is idle. > > > > > > > > So when iterating for idle-cores, if select_idle_core() finds an > > > > idle-core, then it doesn't try to reset this information. > > > > > > > > So if there was only one idle core in the LLC and select_idle_core() > > > > selected the idle-core, the LLC will maintain that it still has a > > > > idle-core. > > > > > > > > On the converse, if a task is pinned, and has a restricted > > > > cpus_allowed_list and LLC has multiple idle-cores, but select_idle_core > > > > cannot find a idle-core, LLC will no more maintain that it has an > > > > idle-core. > > > > > > > > As a first step to solve this problem, LLC will maintain the identity of > > > > the idle core instead of just the information that LLC has an idle core > > > > > > > > Along with maintaining, this change will solve both the problems listed > > > > above. However there are other problems that exist with the current > > > > infrastructure and those will continue to exist with this change and > > > > would be handled in subsequent patches. > > > > > > > > Cc: LKML <linux-kernel@vger.kernel.org> > > > > Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com> > > > > Cc: Parth Shah <parth@linux.ibm.com> > > > > Cc: Ingo Molnar <mingo@kernel.org> > > > > Cc: Peter Zijlstra <peterz@infradead.org> > > > > Cc: Valentin Schneider <valentin.schneider@arm.com> > > > > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com> > > > > Cc: Mel Gorman <mgorman@techsingularity.net> > > > > Cc: Vincent Guittot <vincent.guittot@linaro.org> > > > > Cc: Rik van Riel <riel@surriel.com> > > > > Cc: Aubrey Li <aubrey.li@linux.intel.com> > > > > Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com> > > > > --- > > > > Changelog v2->v3: > > > > - Rebase to tip/sched/core > > > > (Valentin) > > > > > > > > include/linux/sched/topology.h | 2 +- > > > > kernel/sched/fair.c | 52 ++++++++++++++++++---------------- > > > > kernel/sched/sched.h | 3 ++ > > > > kernel/sched/topology.c | 7 +++++ > > > > 4 files changed, 39 insertions(+), 25 deletions(-) > > > > > > > > diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h > > > > index 8f0f778b7c91..285165a35f21 100644 > > > > --- a/include/linux/sched/topology.h > > > > +++ b/include/linux/sched/topology.h > > > > @@ -73,7 +73,7 @@ struct sched_group; > > > > struct sched_domain_shared { > > > > atomic_t ref; > > > > atomic_t nr_busy_cpus; > > > > - int has_idle_cores; > > > > + int idle_core; > > > > }; > > > > > > > > struct sched_domain { > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > > index 7920f2a4d257..c42b2b3cd08f 100644 > > > > --- a/kernel/sched/fair.c > > > > +++ b/kernel/sched/fair.c > > > > @@ -1578,11 +1578,11 @@ numa_type numa_classify(unsigned int imbalance_pct, > > > > > > > > #ifdef CONFIG_SCHED_SMT > > > > /* Forward declarations of select_idle_sibling helpers */ > > > > -static inline bool test_idle_cores(int cpu, bool def); > > > > +static inline int get_idle_core(int cpu, int def); > > > > static inline int numa_idle_core(int idle_core, int cpu) > > > > { > > > > if (!static_branch_likely(&sched_smt_present) || > > > > - idle_core >= 0 || !test_idle_cores(cpu, false)) > > > > + idle_core >= 0 || get_idle_core(cpu, -1) < 0) > > > > return idle_core; > > > > > > > > /* > > > > @@ -6039,29 +6039,31 @@ static inline int __select_idle_cpu(int cpu) > > > > 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 set_idle_core(int cpu, int val) > > > > { > > > > 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->idle_core, val); > > > > } > > > > > > > > -static inline bool test_idle_cores(int cpu, bool def) > > > > +static inline int get_idle_core(int cpu, int def) > > > > { > > > > struct sched_domain_shared *sds; > > > > > > > > - sds = rcu_dereference(per_cpu(sd_llc_shared, cpu)); > > > > - if (sds) > > > > - return READ_ONCE(sds->has_idle_cores); > > > > + if (static_branch_likely(&sched_smt_present)) { > > > > > > Would be good to explain why it is needed to add back the statis branch > > > > > > > Agree, this is not needed. will fix in the next version. > > Thanks for pointing out. > > > > > > + sds = rcu_dereference(per_cpu(sd_llc_shared, cpu)); > > > > + if (sds) > > > > + return READ_ONCE(sds->idle_core); > > > > + } > > > > > > > > return def; > > > > } > > > > > > > > /* > > > > * Scans the local SMT mask to see if the entire core is idle, and records this > > > > - * information in sd_llc_shared->has_idle_cores. > > > > + * information in sd_llc_shared->idle_core. > > > > * > > > > * Since SMT siblings share all cache levels, inspecting this limited remote > > > > * state should be fairly cheap. > > > > @@ -6072,7 +6074,7 @@ void __update_idle_core(struct rq *rq) > > > > int cpu; > > > > > > > > rcu_read_lock(); > > > > - if (test_idle_cores(core, true)) > > > > + if (get_idle_core(core, 0) >= 0) > > > > goto unlock; > > > > > > > > for_each_cpu(cpu, cpu_smt_mask(core)) { > > > > @@ -6083,7 +6085,7 @@ void __update_idle_core(struct rq *rq) > > > > goto unlock; > > > > } > > > > > > > > - set_idle_cores(core, 1); > > > > + set_idle_core(core, per_cpu(smt_id, core)); > > > > unlock: > > > > rcu_read_unlock(); > > > > } > > > > @@ -6091,7 +6093,7 @@ void __update_idle_core(struct rq *rq) > > > > /* > > > > * Scan the entire LLC domain for idle cores; this dynamically switches off if > > > > * there are no idle cores left in the system; tracked through > > > > - * sd_llc->shared->has_idle_cores and enabled through update_idle_core() above. > > > > + * sd_llc->shared->idle_core and enabled through update_idle_core() above. > > > > */ > > > > static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpus, int *idle_cpu) > > > > { > > > > @@ -6144,11 +6146,11 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t > > > > > > > > #else /* CONFIG_SCHED_SMT */ > > > > > > > > -static inline void set_idle_cores(int cpu, int val) > > > > +static inline void set_idle_core(int cpu, int val) > > > > { > > > > } > > > > > > > > -static inline bool test_idle_cores(int cpu, bool def) > > > > +static inline bool get_idle_core(int cpu, int def) > > > > { > > > > return def; > > > > } > > > > @@ -6170,10 +6172,11 @@ static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd > > > > * comparing the average scan cost (tracked in sd->avg_scan_cost) against the > > > > * average idle time for this rq (as found in rq->avg_idle). > > > > */ > > > > -static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool has_idle_core, int target) > > > > +static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int idle_core, int target) > > > > { > > > > struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask); > > > > int i, cpu, idle_cpu = -1, nr = INT_MAX; > > > > + bool has_idle_core = (idle_core != -1); > > > > int this = smp_processor_id(); > > > > struct sched_domain *this_sd; > > > > u64 time; > > > > @@ -6206,8 +6209,13 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool > > > > for_each_cpu_wrap(cpu, cpus, target) { > > > > if (has_idle_core) { > > > > i = select_idle_core(p, cpu, cpus, &idle_cpu); > > > > - if ((unsigned int)i < nr_cpumask_bits) > > > > + if ((unsigned int)i < nr_cpumask_bits) { > > > > +#ifdef CONFIG_SCHED_SMT > > > > + if ((per_cpu(smt_id, i)) == idle_core) > > > > + set_idle_core(i, -1); > > > > +#endif > > > > > > CPUA-core0 enters idle > > > All other CPUs of core0 are already idle > > > set idle_core = core0 > > > CPUB-core1 enters idle > > > All other CPUs of core1 are already idle so core1 becomes idle > > > > > > A task wakes up and select_idle_core returns CPUA-core0 > > > then idle_core=-1 > > > > > > At next wake up, we skip select_idlecore whereas core1 is idle > > > > > > Do I miss something ? > > > > > > > You are right, but this is similar to what we do currently do too. Even > > without this patch, we got ahead an unconditionally (We dont even have an > > option to see if the selected CPU was from an idle-core.) set the idle-core > > to -1. (Please see the hunk I removed below) > > The current race window is limited between select_idle_core() not > finding an idle core and the call of set_idle_cores(this, false); > later in end select_idle_cpu(). >
However lets say there was only one idle-core, and select_idle_core() finds this idle-core, it just doesn't reset has-idle-core. So on the next wakeup, we end up iterating through the whole LLC to find if we have an idle-core.
Also even if there were more than one idle-core in LLC and the task had a limited cpu_allowed_list, and hence had to skip the idle-core, then we still go ahead and reset the idle-core.
> In your proposal, the race is not limited in time anymore. As soon as > the 1st core being idle and setting idle_core is then selected by > select_idle_core, then idle_core is broken >
Yes, but with the next patch, as soon as a CPU within this LLC goes to idle, it will search and set the right idle-core.
> > > > I try to improve upon this in the next iteration. But that again we are > > seeing some higher utilization probably with that change. > > > > I plan to move to a cpumask based approach in v4. > > By which we dont have to search for setting an idle-core but we still know > > if any idle-cores are around. However that will have the extra penalty of > > atomic operations that you commented to in one of my patches. > > > > But if you have other ideas, I would be willing to try out. > > > > > > > > > > > > return i; > > > > + } > > > > > > > > } else { > > > > if (!--nr) > > > > @@ -6218,9 +6226,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool > > > > } > > > > } > > > > > > > > - if (has_idle_core) > > > > - set_idle_cores(this, false); > > > > - > > > > I was referring to this hunk. > > > > > > if (sched_feat(SIS_PROP) && !has_idle_core) { > > > > time = cpu_clock(this) - time; > > > > update_avg(&this_sd->avg_scan_cost, time); > > > > @@ -6276,10 +6281,9 @@ static inline bool asym_fits_capacity(int task_util, int cpu) > > > > */ > > > > static int select_idle_sibling(struct task_struct *p, int prev, int target) > > > > { > > > > - bool has_idle_core = false; > > > > + int i, recent_used_cpu, idle_core = -1; > > > > struct sched_domain *sd; > > > > unsigned long task_util; > > > > - int i, recent_used_cpu; > > > > > > > > /* > > > > * On asymmetric system, update task utilization because we will check > > > > @@ -6357,16 +6361,16 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) > > > > return target; > > > > > > > > if (sched_smt_active()) { > > > > - has_idle_core = test_idle_cores(target, false); > > > > + idle_core = get_idle_core(target, -1); > > > > > > > > - if (!has_idle_core && cpus_share_cache(prev, target)) { > > > > + if (idle_core < 0 && cpus_share_cache(prev, target)) { > > > > i = select_idle_smt(p, sd, prev); > > > > if ((unsigned int)i < nr_cpumask_bits) > > > > return i; > > > > } > > > > } > > > > > > > > - i = select_idle_cpu(p, sd, has_idle_core, target); > > > > + i = select_idle_cpu(p, sd, idle_core, target); > > > > if ((unsigned)i < nr_cpumask_bits) > > > > return i; > > > > > > > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > > > > index a189bec13729..22fbb50b036e 100644 > > > > --- a/kernel/sched/sched.h > > > > +++ b/kernel/sched/sched.h > > > > @@ -1491,6 +1491,9 @@ static inline struct sched_domain *lowest_flag_domain(int cpu, int flag) > > > > DECLARE_PER_CPU(struct sched_domain __rcu *, sd_llc); > > > > DECLARE_PER_CPU(int, sd_llc_size); > > > > DECLARE_PER_CPU(int, sd_llc_id); > > > > +#ifdef CONFIG_SCHED_SMT > > > > +DECLARE_PER_CPU(int, smt_id); > > > > +#endif > > > > DECLARE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared); > > > > DECLARE_PER_CPU(struct sched_domain __rcu *, sd_numa); > > > > DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing); > > > > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c > > > > index 55a0a243e871..232fb261dfc2 100644 > > > > --- a/kernel/sched/topology.c > > > > +++ b/kernel/sched/topology.c > > > > @@ -644,6 +644,9 @@ static void destroy_sched_domains(struct sched_domain *sd) > > > > DEFINE_PER_CPU(struct sched_domain __rcu *, sd_llc); > > > > DEFINE_PER_CPU(int, sd_llc_size); > > > > DEFINE_PER_CPU(int, sd_llc_id); > > > > +#ifdef CONFIG_SCHED_SMT > > > > +DEFINE_PER_CPU(int, smt_id); > > > > +#endif > > > > DEFINE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared); > > > > DEFINE_PER_CPU(struct sched_domain __rcu *, sd_numa); > > > > DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing); > > > > @@ -667,6 +670,9 @@ static void update_top_cache_domain(int cpu) > > > > rcu_assign_pointer(per_cpu(sd_llc, cpu), sd); > > > > per_cpu(sd_llc_size, cpu) = size; > > > > per_cpu(sd_llc_id, cpu) = id; > > > > +#ifdef CONFIG_SCHED_SMT > > > > + per_cpu(smt_id, cpu) = cpumask_first(cpu_smt_mask(cpu)); > > > > +#endif > > > > rcu_assign_pointer(per_cpu(sd_llc_shared, cpu), sds); > > > > > > > > sd = lowest_flag_domain(cpu, SD_NUMA); > > > > @@ -1497,6 +1503,7 @@ sd_init(struct sched_domain_topology_level *tl, > > > > sd->shared = *per_cpu_ptr(sdd->sds, sd_id); > > > > atomic_inc(&sd->shared->ref); > > > > atomic_set(&sd->shared->nr_busy_cpus, sd_weight); > > > > + sd->shared->idle_core = -1; > > > > } > > > > > > > > sd->private = sdd; > > > > -- > > > > 2.18.2 > > > > > > > > -- > > Thanks and Regards > > Srikar Dronamraju
-- Thanks and Regards Srikar Dronamraju
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