Re: [PATCH v5 7/7] sched/fair: Add latency list

[Vincent Guittot <vincent.guittot@linaro.org>]

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@linaro.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@linaro.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 ?

---
 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
> > > >
> > > >
> > > >