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Date: Fri, 14 Oct 2022 17:22:35 +0200 From: Vincent Guittot <vincent.guittot@...aro.org> To: Youssef Esmat <youssefesmat@...omium.org> Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com, linux-kernel@...r.kernel.org, parth@...ux.ibm.com, qais.yousef@....com, chris.hyser@...cle.com, valentin.schneider@....com, patrick.bellasi@...bug.net, David.Laight@...lab.com, pjt@...gle.com, pavel@....cz, tj@...nel.org, qperret@...gle.com, tim.c.chen@...ux.intel.com, joshdon@...gle.com, timj@....org, joel@...lfernandes.org Subject: Re: [PATCH v5 7/7] sched/fair: Add latency list On Thu, 13 Oct 2022 at 19:19, Youssef Esmat <youssefesmat@...omium.org> wrote: > > On Wed, Oct 12, 2022 at 10:21 AM Vincent Guittot > <vincent.guittot@...aro.org> wrote: > > > > Le mardi 11 oct. 2022 à 18:54:27 (-0500), Youssef Esmat a écrit : > > > Hi Vincent, > > > > > > On Tue, Oct 11, 2022 at 12:10 PM Vincent Guittot > > > <vincent.guittot@...aro.org> wrote: > > > > ... > > > > > > > > latency 0 latency -20 > > > > > > Min Latencies: 60 61 > > > > > > Avg Latencies: 1077 86 > > > > > > Max Latencies: 87311 444 > > > > > > 50% latencies: 92 85 > > > > > > 75% latencies: 554 90 > > > > > > 85% latencies: 1019 93 > > > > > > 90% latencies: 1346 96 > > > > > > 95% latencies: 5400 100 > > > > > > 99% latencies: 19044 110 > > > > > > > > > > > > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org> > > > > > > --- > > > > > > > > > > The ability to boost the latency sensitivity of a task seems very > > > > > interesting. I have been playing around with these changes and have > > > > > some observations. > > > > > > > > > > I tried 2 bursty tasks affinitized to the same CPU. The tasks sleep > > > > > for 1ms and run for 10ms in a loop. I first tried it without adjusting > > > > > the latency_nice value and took perf sched traces: > > > > > > > > The CPU is overloaded almost all the time as it can't run the 2 tasks > > > > (2*10ms every 11ms) > > > > > > > > > > > > > > latency_test:7040 | 2447.137 ms | 8 | avg: 6.546 ms | > > > > > max: 10.674 ms | max start: 353.809487 s | max end: 353.820161 s > > > > > latency_test:7028 | 2454.777 ms | 7 | avg: 4.494 ms | > > > > > max: 10.609 ms | max start: 354.804386 s | max end: 354.814995 s > > > > > > > > > > Everything looked as expected, for a 5s run they had similar runtime > > > > > and latency. > > > > > > > > > > I then adjusted one task to have a latency_nice of -20 (pid 8614 > > > > > below) and took another set of traces: > > > > > > > > > > latency_test:8618 | 1845.534 ms | 131 | avg: 9.764 ms | > > > > > max: 10.686 ms | max start: 1405.737905 s | max end: 1405.748592 s > > > > > latency_test:8614 | 3033.635 ms | 16 | avg: 3.559 ms | > > > > > max: 10.467 ms | max start: 1407.594751 s | max end: 1407.605218 s > > > > > > > > > > The task with -20 latency_nice had significantly more runtime. The > > > > > average latency was improved but the max roughly stayed the same. As > > > > > expected the one with latency_nice value of 0 experienced more > > > > > switches, but so did the one with latency_nice of -20. > > > > > > > > Your results look unexpected because the vruntime of the tasks is not > > > > modified. So I can imagine that the thread with the low latency runs > > > > first up to the offset at the beg of the test but then they should > > > > switch regularly. I have tried a similar test with a modified rt-app > > > > and the result seems ok. I have a small difference but not the > > > > difference that you see. > > > > > > > > Could you share more details about your setup ? I'm going to try to > > > > reproduce your sequence > > > > > > I was using an intel core i7 with this frequency details: > > > CPU MHz: 4200.000 > > > CPU max MHz: 4800.0000 > > > CPU min MHz: 400.0000 > > > > > > This is a snippet of the test I was using: > > > > > > struct sched_attr attr; > > > memset(&attr, 0, sizeof(struct sched_attr)); > > > attr.size = sizeof(struct sched_attr); > > > attr.sched_latency_nice = nice_latency; > > > attr.sched_flags = SCHED_FLAG_LATENCY_NICE; > > > > > > // set nice latency value > > > int res = syscall(__NR_sched_setattr, 0, &attr, 0); > > > > > > while(1){ > > > // wake up every ms > > > usleep(1000); > > > for(int i = 0; i < 40000000; i++){} > > > } > > > > Between v2 and v3, the sched_feat(GENTLE_FAIR_SLEEPERS) has diseappeared when > > I moved the computation of latency_offset at the setting of the latency prio > > instead of runtime. As a result, the latency nice task can preempt up to > > (sysctl_sched_latency - sysctl_sched_wakeup_granularity) but other threads are > > cap to half of sysctl_sched_latency at wakeup. > > > > Could you try the patch below ? > > Thanks Vincent. That improved the runtime for the tests above. > > Running two bursty tasks (1ms sleep and 10ms running loop) one of > which had a latency_nice of -20 (pid 9209) and the other 0: > > latency_test:9205 | 2429.467 ms | 14 | avg: 6.293 ms | > max: 11.340 ms | max start: 209.543482 s | max end: 209.554821 s > latency_test:9209 | 2467.423 ms | 17 | avg: 3.346 ms | > max: 11.475 ms | max start: 208.374517 s | max end: 208.385992 s > > The task with -20 latency_nice had an improved average latency, but > the max roughly stayed the same. This is expected because there is not enough cpu time to run both tasks in your UC. Task with -20 latency will run 1st whenever it has not used all its bandwidth but a some point, It will have to let the other task runs 1st to not get an unfair amount of cpu bandwidth TL is task with -20 latency TO is the ther task TO wakes up and start to run After 3ms TL wakes up TL preempts TO because 3ms delta +12ms latency offset > 4ms sched_akeup_granularity TL runs for 10ms then go to sleep TL vruntime is +7ms vs TO TO starts to run 1ms later TL wakeup TL preempts TO because delta -6ms delta +12ms latency offset > 4ms sched_akeup_granularity TL runs for 10ms then go to sleep TL vruntime is +16ms vs TO TO starts to run 1ms later TL wakeup TL can't preempt TO because delta -15ms delta +12ms latency offset < 4ms sched_wakeup_granularity TL has exhausted its cpu bandwidth and will wait like other cfs task I will look more deeply on the use cases below > > Running the same test with 3 bursty tasks, one of which had a > latency_nice of -20 and the other two with latency_nice = 0. The > results for this case roughly stayed the same after the changes: > > latency_test:26088 | 1641.458 ms | 158 | avg: 19.613 ms | > max: 33.000 ms | max start: 871.707231 s | max end: 871.740231 s > latency_test:26295 | 1639.766 ms | 238 | avg: 10.259 ms | > max: 24.289 ms | max start: 873.917231 s | max end: 873.941519 s > latency_test:26401 | 1643.580 ms | 241 | avg: 10.200 ms | > max: 22.124 ms | max start: 876.289233 s | max end: 876.311357 s > > The task with latency_nice -20 seemed to have the highest average and > max latencies. > > I also tried an additional test where we had a task that was cpu bound > (while(1){}) and a task that ran for 1ms and slept for 1ms. The cpu > bound task had the latency_nice value of -20 and the bursty had > latency_nice of 0. > > latency_test:17353 | 4557.699 ms | 356 | avg: 1.058 ms | > max: 1.181 ms | max start: 4324.806123 s | max end: 4324.807304 s > latency_test:17452 | 377.804 ms | 1 | avg: 0.000 ms | > max: 0.000 ms | max start: 0.000000 s | max end: 0.000000 s > > The cpu bound task (pid 17353 above) had significantly more runtime. > If I reran the test with latency_nice = 0 for both: > > latency_test:20748 | 2478.014 ms | 2367 | avg: 1.037 ms | > max: 1.182 ms | max start: 4460.769240 s | max end: 4460.770423 s > latency_test:20972 | 2455.888 ms | 3 | avg: 0.001 ms | > max: 0.002 ms | max start: 4460.628756 s | max end: 4460.628758 s > > Would decreasing the latency_offset as the task runs and increasing it > as it sleeps help here? > > > > > > --- > > kernel/sched/core.c | 19 ++++++++++++++++--- > > 1 file changed, 16 insertions(+), 3 deletions(-) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 8b44685ae247..68f9a83d7089 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -1287,11 +1287,15 @@ static void set_load_weight(struct task_struct *p, bool update_load) > > static void set_latency_offset(struct task_struct *p) > > { > > long weight = sched_latency_to_weight[p->latency_prio]; > > + unsigned long period = sysctl_sched_latency; > > s64 offset; > > > > - offset = sysctl_sched_latency * weight; > > + if (sched_feat(GENTLE_FAIR_SLEEPERS)) > > + period >>= 1; > > + offset = period * weight; > > offset = div_s64(offset, NICE_LATENCY_WEIGHT_MAX); > > p->se.latency_offset = (long)offset; > > + trace_printk("set_latency_offset pid %d offset %ld", p->pid, offset); > > } > > > > #ifdef CONFIG_UCLAMP_TASK > > @@ -10894,12 +10898,17 @@ static int cpu_idle_write_s64(struct cgroup_subsys_state *css, > > static s64 cpu_latency_nice_read_s64(struct cgroup_subsys_state *css, > > struct cftype *cft) > > { > > + unsigned long period = sysctl_sched_latency; > > int last_delta = INT_MAX; > > int prio, delta; > > s64 weight; > > > > + if (sched_feat(GENTLE_FAIR_SLEEPERS)) > > + period >>= 1; > > + > > weight = css_tg(css)->latency_offset * NICE_LATENCY_WEIGHT_MAX; > > - weight = div_s64(weight, sysctl_sched_latency); > > + period = sysctl_sched_latency; > > + weight = div_s64(weight, period); > > > > /* Find the closest nice value to the current weight */ > > for (prio = 0; prio < ARRAY_SIZE(sched_latency_to_weight); prio++) { > > @@ -10915,6 +10924,7 @@ static s64 cpu_latency_nice_read_s64(struct cgroup_subsys_state *css, > > static int cpu_latency_nice_write_s64(struct cgroup_subsys_state *css, > > struct cftype *cft, s64 nice) > > { > > + unsigned long period; > > s64 latency_offset; > > long weight; > > int idx; > > @@ -10926,7 +10936,10 @@ static int cpu_latency_nice_write_s64(struct cgroup_subsys_state *css, > > idx = array_index_nospec(idx, LATENCY_NICE_WIDTH); > > weight = sched_latency_to_weight[idx]; > > > > - latency_offset = sysctl_sched_latency * weight; > > + period = sysctl_sched_latency; > > + if (sched_feat(GENTLE_FAIR_SLEEPERS)) > > + period >>= 1; > > + latency_offset = period * weight; > > latency_offset = div_s64(latency_offset, NICE_LATENCY_WEIGHT_MAX); > > > > return sched_group_set_latency(css_tg(css), latency_offset); > > -- > > 2.17.1 > > > > > > > > > > > > > > > > > > > Also tried running the same test but instead of using latency nice I > > > > > adjusted the nice value as a comparison. In that case one task had a > > > > > nice of -5 and the other was 0. > > > > > > > > > > nice_test:25219 | 1216.839 ms | 242 | avg: 10.295 ms | > > > > > max: 11.927 ms | max start: 5877.881279 s | max end: 5877.893206 s > > > > > nice_test:25235 | 3711.788 ms | 6 | avg: 1.026 ms | > > > > > max: 6.143 ms | max start: 5875.603741 s | max end: 5875.609883 s > > > > > > > > > > As expected the one with a nice value of -5 had more runtime but also > > > > > had better latency numbers than in the previous case of using > > > > > latency_nice. > > > > > > > > > > I also tried a similar test with 3 bursty tasks instead of two. In > > > > > this case all tasks had a latency_nice of 0: > > > > > > > > > > latency_test:11467 | 1641.131 ms | 161 | avg: 17.489 ms | > > > > > max: 21.011 ms | max start: 1542.656275 s | max end: 1542.677286 s > > > > > latency_test:11463 | 1644.809 ms | 161 | avg: 11.994 ms | > > > > > max: 25.012 ms | max start: 1545.657776 s | max end: 1545.682788 s > > > > > latency_test:11478 | 1643.211 ms | 160 | avg: 11.465 ms | > > > > > max: 21.012 ms | max start: 1546.159026 s | max end: 1546.180038 s > > > > > > > > > > Next I tried two tasks with a latency_nice of 0 and a third one had a > > > > > latency_nice of -20 (pid 11763 below): > > > > > > > > > > latency_test:11763 | 1645.482 ms | 159 | avg: 19.634 ms | > > > > > max: 31.016 ms | max start: 1623.834862 s | max end: 1623.865877 s > > > > > latency_test:11750 | 1644.276 ms | 259 | avg: 9.985 ms | > > > > > max: 21.012 ms | max start: 1623.953921 s | max end: 1623.974933 s > > > > > latency_test:11747 | 1642.745 ms | 262 | avg: 9.079 ms | > > > > > max: 25.013 ms | max start: 1620.980435 s | max end: 1621.005447 s > > > > > > > > > > In this case it seemed like the runtime was not affected by the > > > > > latency_nice value, but strangely the task with the latency nice of > > > > > -20 had a worse average and max latency than the other two. The > > > > > context switch times are also increased from the previous case. > > > > > > > > > > Have we considered an approach where the task that is marked as > > > > > latency sensitive gets a boosted nice value when it sleeps and is > > > > > either scaled down exponentially as it runs, or immediately reset to > > > > > its default when it runs for one tick? > > > > > > > > > > Thanks, > > > > > Youssef > > > > > > > > > > > > > > > > include/linux/sched.h | 2 + > > > > > > kernel/sched/fair.c | 96 +++++++++++++++++++++++++++++++++++++++++-- > > > > > > kernel/sched/sched.h | 1 + > > > > > > 3 files changed, 96 insertions(+), 3 deletions(-) > > > > > > > > > > > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > > > > > index a74cad08e91e..0b92674e3664 100644 > > > > > > --- a/include/linux/sched.h > > > > > > +++ b/include/linux/sched.h > > > > > > @@ -547,6 +547,8 @@ struct sched_entity { > > > > > > /* For load-balancing: */ > > > > > > struct load_weight load; > > > > > > struct rb_node run_node; > > > > > > + struct rb_node latency_node; > > > > > > + unsigned int on_latency; > > > > > > struct list_head group_node; > > > > > > unsigned int on_rq; > > > > > > > > > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > > > > index e524e892d118..1a72f34136d8 100644 > > > > > > --- a/kernel/sched/fair.c > > > > > > +++ b/kernel/sched/fair.c > > > > > > @@ -664,7 +664,77 @@ struct sched_entity *__pick_last_entity(struct cfs_rq *cfs_rq) > > > > > > > > > > > > return __node_2_se(last); > > > > > > } > > > > > > +#endif > > > > > > > > > > > > +/************************************************************** > > > > > > + * Scheduling class tree data structure manipulation methods: > > > > > > + * for latency > > > > > > + */ > > > > > > + > > > > > > +static inline bool latency_before(struct sched_entity *a, > > > > > > + struct sched_entity *b) > > > > > > +{ > > > > > > + return (s64)(a->vruntime + a->latency_offset - b->vruntime - b->latency_offset) < 0; > > > > > > +} > > > > > > + > > > > > > +#define __latency_node_2_se(node) \ > > > > > > + rb_entry((node), struct sched_entity, latency_node) > > > > > > + > > > > > > +static inline bool __latency_less(struct rb_node *a, const struct rb_node *b) > > > > > > +{ > > > > > > + return latency_before(__latency_node_2_se(a), __latency_node_2_se(b)); > > > > > > +} > > > > > > + > > > > > > +/* > > > > > > + * Enqueue an entity into the latency rb-tree: > > > > > > + */ > > > > > > +static void __enqueue_latency(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) > > > > > > +{ > > > > > > + > > > > > > + /* Only latency sensitive entity can be added to the list */ > > > > > > + if (se->latency_offset >= 0) > > > > > > + return; > > > > > > + > > > > > > + if (se->on_latency) > > > > > > + return; > > > > > > + > > > > > > + /* > > > > > > + * An execution time less than sysctl_sched_min_granularity means that > > > > > > + * the entity has been preempted by a higher sched class or an entity > > > > > > + * with higher latency constraint. > > > > > > + * Put it back in the list so it gets a chance to run 1st during the > > > > > > + * next slice. > > > > > > + */ > > > > > > + if (!(flags & ENQUEUE_WAKEUP)) { > > > > > > + u64 delta_exec = se->sum_exec_runtime - se->prev_sum_exec_runtime; > > > > > > + > > > > > > + if (delta_exec >= sysctl_sched_min_granularity) > > > > > > + return; > > > > > > + } > > > > > > + > > > > > > + rb_add_cached(&se->latency_node, &cfs_rq->latency_timeline, __latency_less); > > > > > > + se->on_latency = 1; > > > > > > +} > > > > > > + > > > > > > +static void __dequeue_latency(struct cfs_rq *cfs_rq, struct sched_entity *se) > > > > > > +{ > > > > > > + if (se->on_latency) { > > > > > > + rb_erase_cached(&se->latency_node, &cfs_rq->latency_timeline); > > > > > > + se->on_latency = 0; > > > > > > + } > > > > > > +} > > > > > > + > > > > > > +static struct sched_entity *__pick_first_latency(struct cfs_rq *cfs_rq) > > > > > > +{ > > > > > > + struct rb_node *left = rb_first_cached(&cfs_rq->latency_timeline); > > > > > > + > > > > > > + if (!left) > > > > > > + return NULL; > > > > > > + > > > > > > + return __latency_node_2_se(left); > > > > > > +} > > > > > > + > > > > > > +#ifdef CONFIG_SCHED_DEBUG > > > > > > /************************************************************** > > > > > > * Scheduling class statistics methods: > > > > > > */ > > > > > > @@ -4455,8 +4525,10 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) > > > > > > check_schedstat_required(); > > > > > > update_stats_enqueue_fair(cfs_rq, se, flags); > > > > > > check_spread(cfs_rq, se); > > > > > > - if (!curr) > > > > > > + if (!curr) { > > > > > > __enqueue_entity(cfs_rq, se); > > > > > > + __enqueue_latency(cfs_rq, se, flags); > > > > > > + } > > > > > > se->on_rq = 1; > > > > > > > > > > > > if (cfs_rq->nr_running == 1) { > > > > > > @@ -4542,8 +4614,10 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) > > > > > > > > > > > > clear_buddies(cfs_rq, se); > > > > > > > > > > > > - if (se != cfs_rq->curr) > > > > > > + if (se != cfs_rq->curr) { > > > > > > __dequeue_entity(cfs_rq, se); > > > > > > + __dequeue_latency(cfs_rq, se); > > > > > > + } > > > > > > se->on_rq = 0; > > > > > > account_entity_dequeue(cfs_rq, se); > > > > > > > > > > > > @@ -4631,6 +4705,7 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se) > > > > > > */ > > > > > > update_stats_wait_end_fair(cfs_rq, se); > > > > > > __dequeue_entity(cfs_rq, se); > > > > > > + __dequeue_latency(cfs_rq, se); > > > > > > update_load_avg(cfs_rq, se, UPDATE_TG); > > > > > > } > > > > > > > > > > > > @@ -4669,7 +4744,7 @@ static struct sched_entity * > > > > > > pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr) > > > > > > { > > > > > > struct sched_entity *left = __pick_first_entity(cfs_rq); > > > > > > - struct sched_entity *se; > > > > > > + struct sched_entity *latency, *se; > > > > > > > > > > > > /* > > > > > > * If curr is set we have to see if its left of the leftmost entity > > > > > > @@ -4711,6 +4786,12 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr) > > > > > > se = cfs_rq->last; > > > > > > } > > > > > > > > > > > > + /* Check for latency sensitive entity waiting for running */ > > > > > > + latency = __pick_first_latency(cfs_rq); > > > > > > + if (latency && (latency != se) && > > > > > > + wakeup_preempt_entity(latency, se) < 1) > > > > > > + se = latency; > > > > > > + > > > > > > return se; > > > > > > } > > > > > > > > > > > > @@ -4734,6 +4815,7 @@ static void put_prev_entity(struct cfs_rq *cfs_rq, struct sched_entity *prev) > > > > > > update_stats_wait_start_fair(cfs_rq, prev); > > > > > > /* Put 'current' back into the tree. */ > > > > > > __enqueue_entity(cfs_rq, prev); > > > > > > + __enqueue_latency(cfs_rq, prev, 0); > > > > > > /* in !on_rq case, update occurred at dequeue */ > > > > > > update_load_avg(cfs_rq, prev, 0); > > > > > > } > > > > > > @@ -11717,6 +11799,7 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first) > > > > > > void init_cfs_rq(struct cfs_rq *cfs_rq) > > > > > > { > > > > > > cfs_rq->tasks_timeline = RB_ROOT_CACHED; > > > > > > + cfs_rq->latency_timeline = RB_ROOT_CACHED; > > > > > > u64_u32_store(cfs_rq->min_vruntime, (u64)(-(1LL << 20))); > > > > > > #ifdef CONFIG_SMP > > > > > > raw_spin_lock_init(&cfs_rq->removed.lock); > > > > > > @@ -12025,8 +12108,15 @@ int sched_group_set_latency(struct task_group *tg, s64 latency) > > > > > > > > > > > > for_each_possible_cpu(i) { > > > > > > struct sched_entity *se = tg->se[i]; > > > > > > + struct rq *rq = cpu_rq(i); > > > > > > + struct rq_flags rf; > > > > > > + > > > > > > + rq_lock_irqsave(rq, &rf); > > > > > > > > > > > > + __dequeue_latency(se->cfs_rq, se); > > > > > > WRITE_ONCE(se->latency_offset, latency); > > > > > > + > > > > > > + rq_unlock_irqrestore(rq, &rf); > > > > > > } > > > > > > > > > > > > mutex_unlock(&shares_mutex); > > > > > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > > > > > > index a15fb955092c..76bca172585c 100644 > > > > > > --- a/kernel/sched/sched.h > > > > > > +++ b/kernel/sched/sched.h > > > > > > @@ -599,6 +599,7 @@ struct cfs_rq { > > > > > > #endif > > > > > > > > > > > > struct rb_root_cached tasks_timeline; > > > > > > + struct rb_root_cached latency_timeline; > > > > > > > > > > > > /* > > > > > > * 'curr' points to currently running entity on this cfs_rq. > > > > > > -- > > > > > > 2.17.1 > > > > > > > > > > > > > > > > > >
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