Re: [RFC 5/6] sched/fair: Take into account latency nice at wakeup

From: Vincent Guittot
Date: Tue May 03 2022 - 08:40:40 EST

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On Tue, 3 May 2022 at 04:29, Tao Zhou <tao.zhou@xxxxxxxxx> wrote:
>
> On Sun, May 01, 2022 at 11:58:42PM +0800, Tao Zhou wrote:
> > Hi Vincent,
> >
> > Change to Valentin Schneider's now using mail address.
> >
> > On Fri, Mar 11, 2022 at 05:14:05PM +0100, Vincent Guittot wrote:
> >
> > > Take into account the nice latency priority of a thread when deciding to
> > > preempt the current running thread. We don't want to provide more CPU
> > > bandwidth to a thread but reorder the scheduling to run latency sensitive
> > > task first whenever possible.
> > >
> > > As long as a thread didn't use its bandwidth, it will be able to preempt
> > > the current thread.
> > >
> > > At the opposite, a thread with a low latency priority will preempt current
> > > thread at wakeup only to keep fair CPU bandwidth sharing. Otherwise it will
> > > wait for the tick to get its sched slice.
> > >
> > > curr vruntime
> > > |
> > > sysctl_sched_wakeup_granularity
> > > <-->
> > > ----------------------------------|----|-----------------------|---------------
> > > | |<--------------------->
> > > | . sysctl_sched_latency
> > > | .
> > > default/current latency entity | .
> > > | .
> > > 1111111111111111111111111111111111|0000|-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-
> > > se preempts curr at wakeup ------>|<- se doesn't preempt curr -----------------
> > > | .
> > > | .
> > > | .
> > > low latency entity | .
> > > ---------------------->|
> > > % of sysctl_sched_latency |
> > > 1111111111111111111111111111111111111111111111111111111111|0000|-1-1-1-1-1-1-1-
> > > preempt ------------------------------------------------->|<- do not preempt --
> > > | .
> > > | .
> > > | .
> > > high latency entity | .
> > > |<-----------------------| .
> > > | % of sysctl_sched_latency .
> > > 111111111|0000|-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1
> > > preempt->|<- se doesn't preempt curr ------------------------------------------
> > >
> > > Tests results of nice latency impact on heavy load like hackbench:
> > >
> > > hackbench -l (2560 / group) -g group
> > > group latency 0 latency 19
> > > 1 1.450(+/- 0.60%) 1.376(+/- 0.54%) + 5%
> > > 4 1.537(+/- 1.75%) 1.335(+/- 1.81%) +13%
> > > 8 1.414(+/- 1.91%) 1.348(+/- 1.88%) + 5%
> > > 16 1.423(+/- 1.65%) 1.374(+/- 0.58%) + 3%
> > >
> > > hackbench -p -l (2560 / group) -g group
> > > group
> > > 1 1.416(+/- 3.45%) 0.886(+/- 0.54%) +37%
> > > 4 1.634(+/- 7.14%) 0.888(+/- 5.40%) +45%
> > > 8 1.449(+/- 2.14%) 0.804(+/- 4.55%) +44%
> > > 16 0.917(+/- 4.12%) 0.777(+/- 1.41%) +15%
> > >
> > > By deacreasing the latency prio, we reduce the number of preemption at
> >
> > s/deacreasing/decreasing/
> > s/reduce/increase/
> >
> > > wakeup and help hackbench making progress.
> > >
> > > Test results of nice latency impact on short live load like cyclictest
> > > while competing with heavy load like hackbench:
> > >
> > > hackbench -l 10000 -g group &
> > > cyclictest --policy other -D 5 -q -n
> > > latency 0 latency -20
> > > group min avg max min avg max
> > > 0 16 17 28 15 17 27
> > > 1 61 382 10603 63 89 4628
> > > 4 52 437 15455 54 98 16238
> > > 8 56 728 38499 61 125 28983
> > > 16 53 1215 52207 61 183 80751
> > >
> > > group = 0 means that hackbench is not running.
> > >
> > > The avg is significantly improved with nice latency -20 especially with
> > > large number of groups but min and max remain quite similar. If we add the
> > > histogram parameters to get details of latency, we have :
> > >
> > > hackbench -l 10000 -g 16 &
> > > cyclictest --policy other -D 5 -q -n -H 20000 --histfile data.txt
> > > latency 0 latency -20
> > > Min Latencies: 63 62
> > > Avg Latencies: 1397 218
> > > Max Latencies: 44926 42291
> > > 50% latencies: 100 98
> > > 75% latencies: 762 116
> > > 85% latencies: 1118 126
> > > 90% latencies: 1540 130
> > > 95% latencies: 5610 138
> > > 99% latencies: 13738 266
> > >
> > > With percentile details, we see the benefit of nice latency -20 as
> > > 1% of the latencies stays above 266us whereas the default latency has
> > > got 25% are above 762us and 10% over the 1ms.
> > >
> > > Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > > ---
> > > include/linux/sched.h | 4 ++-
> > > init/init_task.c | 2 +-
> > > kernel/sched/core.c | 32 +++++++++++++++++++----
> > > kernel/sched/debug.c | 2 +-
> > > kernel/sched/fair.c | 60 +++++++++++++++++++++++++++++++++++++++++--
> > > kernel/sched/sched.h | 12 +++++++++
> > > 6 files changed, 102 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > index 2aa889a59054..9aeb157e819b 100644
> > > --- a/include/linux/sched.h
> > > +++ b/include/linux/sched.h
> > > @@ -560,6 +560,8 @@ struct sched_entity {
> > > unsigned long runnable_weight;
> > > #endif
> > >
> > > + int latency_weight;
> > > +
> > > #ifdef CONFIG_SMP
> > > /*
> > > * Per entity load average tracking.
> > > @@ -779,7 +781,7 @@ struct task_struct {
> > > int static_prio;
> > > int normal_prio;
> > > unsigned int rt_priority;
> > > - int latency_nice;
> > > + int latency_prio;
> > >
> > > struct sched_entity se;
> > > struct sched_rt_entity rt;
> > > diff --git a/init/init_task.c b/init/init_task.c
> > > index 2afa249c253b..e98c71f24981 100644
> > > --- a/init/init_task.c
> > > +++ b/init/init_task.c
> > > @@ -78,7 +78,7 @@ struct task_struct init_task
> > > .prio = MAX_PRIO - 20,
> > > .static_prio = MAX_PRIO - 20,
> > > .normal_prio = MAX_PRIO - 20,
> > > - .latency_nice = 0,
> > > + .latency_prio = NICE_WIDTH - 20,
> > > .policy = SCHED_NORMAL,
> > > .cpus_ptr = &init_task.cpus_mask,
> > > .user_cpus_ptr = NULL,
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 8f8b102a75c4..547b0da01efe 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -1241,6 +1241,11 @@ static void set_load_weight(struct task_struct *p, bool update_load)
> > > }
> > > }
> > >
> > > +static void set_latency_weight(struct task_struct *p)
> > > +{
> > > + p->se.latency_weight = sched_latency_to_weight[p->latency_prio];
> > > +}
> > > +
> > > #ifdef CONFIG_UCLAMP_TASK
> > > /*
> > > * Serializes updates of utilization clamp values
> > > @@ -4394,7 +4399,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
> > > p->prio = current->normal_prio;
> > >
> > > /* Propagate the parent's latency requirements to the child as well */
> > > - p->latency_nice = current->latency_nice;
> > > + p->latency_prio = current->latency_prio;
> > >
> > > uclamp_fork(p);
> > >
> > > @@ -4412,7 +4417,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
> > > p->prio = p->normal_prio = p->static_prio;
> > > set_load_weight(p, false);
> > >
> > > - p->latency_nice = DEFAULT_LATENCY_NICE;
> > > + p->latency_prio = NICE_TO_LATENCY(0);
> > > /*
> > > * We don't need the reset flag anymore after the fork. It has
> > > * fulfilled its duty:
> > > @@ -4420,6 +4425,9 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
> > > p->sched_reset_on_fork = 0;
> > > }
> > >
> > > + /* Once latency_prio is set, update the latency weight */
> > > + set_latency_weight(p);
> > > +
> > > if (dl_prio(p->prio))
> > > return -EAGAIN;
> > > else if (rt_prio(p->prio))
> > > @@ -7361,7 +7369,7 @@ static int __sched_setscheduler(struct task_struct *p,
> > > if (attr->sched_latency_nice < MIN_LATENCY_NICE)
> > > return -EINVAL;
> > > /* Use the same security checks as NICE */
> > > - if (attr->sched_latency_nice < p->latency_nice &&
> > > + if (attr->sched_latency_nice < LATENCY_TO_NICE(p->latency_prio) &&
> > > !capable(CAP_SYS_NICE))
> > > return -EPERM;
> > > }
> > > @@ -7401,7 +7409,7 @@ static int __sched_setscheduler(struct task_struct *p,
> > > if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
> > > goto change;
> > > if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE &&
> > > - attr->sched_latency_nice != p->latency_nice)
> > > + attr->sched_latency_nice != LATENCY_TO_NICE(p->latency_prio))
> > > goto change;
> > >
> > > p->sched_reset_on_fork = reset_on_fork;
> > > @@ -7942,7 +7950,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
> > > get_params(p, &kattr);
> > > kattr.sched_flags &= SCHED_FLAG_ALL;
> > >
> > > - kattr.sched_latency_nice = p->latency_nice;
> > > + kattr.sched_latency_nice = LATENCY_TO_NICE(p->latency_prio);
> > >
> > > #ifdef CONFIG_UCLAMP_TASK
> > > /*
> > > @@ -10954,6 +10962,20 @@ const u32 sched_prio_to_wmult[40] = {
> > > /* 15 */ 119304647, 148102320, 186737708, 238609294, 286331153,
> > > };
> > >
> > > +/*
> > > + * latency weight for wakeup preemption
> > > + */
> > > +const int sched_latency_to_weight[40] = {
> > > + /* -20 */ 1024, 973, 922, 870, 819,
> > > + /* -15 */ 768, 717, 666, 614, 563,
> > > + /* -10 */ 512, 461, 410, 358, 307,
> > > + /* -5 */ 256, 205, 154, 102, 51,
> > > + /* 0 */ 0, -51, -102, -154, -205,
> > > + /* 5 */ -256, -307, -358, -410, -461,
> > > + /* 10 */ -512, -563, -614, -666, -717,
> > > + /* 15 */ -768, -819, -870, -922, -973,
> > > +};
> > > +
> > > void call_trace_sched_update_nr_running(struct rq *rq, int count)
> > > {
> > > trace_sched_update_nr_running_tp(rq, count);
> > > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> > > index 5d76a8927888..253e52ec73fb 100644
> > > --- a/kernel/sched/debug.c
> > > +++ b/kernel/sched/debug.c
> > > @@ -1043,7 +1043,7 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
> > > #endif
> > > P(policy);
> > > P(prio);
> > > - P(latency_nice);
> > > + P(latency_prio);
> > > if (task_has_dl_policy(p)) {
> > > P(dl.runtime);
> > > P(dl.deadline);
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 5c4bfffe8c2c..506c482a0e48 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -5555,6 +5555,35 @@ static int sched_idle_cpu(int cpu)
> > > }
> > > #endif
> > >
> > > +static void set_next_buddy(struct sched_entity *se);
> > > +
> > > +static void check_preempt_from_idle(struct cfs_rq *cfs, struct sched_entity *se)
> > > +{
> > > + struct sched_entity *next;
> > > +
> > > + if (se->latency_weight <= 0)
> > > + return;
> > > +
> > > + if (cfs->nr_running <= 1)
> > > + return;
> > > + /*
> > > + * When waking from idle, we don't need to check to preempt at wakeup
> > > + * the idle thread and don't set next buddy as a candidate for being
> > > + * picked in priority.
> > > + * In case of simultaneous wakeup from idle, the latency sensitive tasks
> > > + * lost opportunity to preempt non sensitive tasks which woke up
> > > + * simultaneously.
> > > + */
> > > +
> > > + if (cfs->next)
> > > + next = cfs->next;
> > > + else
> > > + next = __pick_first_entity(cfs);
> > > +
> > > + if (next && wakeup_preempt_entity(next, se) == 1)
> > > + set_next_buddy(se);
> > > +}
> > > +
> > > /*
> > > * The enqueue_task method is called before nr_running is
> > > * increased. Here we update the fair scheduling stats and
> > > @@ -5648,6 +5677,9 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > > if (!task_new)
> > > update_overutilized_status(rq);
> > >
> > > + if (rq->curr == rq->idle)
> > > + check_preempt_from_idle(cfs_rq_of(&p->se), &p->se);
> > > +
> > > enqueue_throttle:
> > > if (cfs_bandwidth_used()) {
> > > /*
> > > @@ -5669,8 +5701,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > > hrtick_update(rq);
> > > }
> > >
> > > -static void set_next_buddy(struct sched_entity *se);
> > > -
> > > /*
> > > * The dequeue_task method is called before nr_running is
> > > * decreased. We remove the task from the rbtree and
> > > @@ -6970,6 +7000,27 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> > > }
> > > #endif /* CONFIG_SMP */
> > >
> > > +static long wakeup_latency_gran(int latency_weight)
> > > +{
> > > + long thresh = sysctl_sched_latency;
> > > +
> > > + if (!latency_weight)
> > > + return 0;
> > > +
> > > + if (sched_feat(GENTLE_FAIR_SLEEPERS))
> > > + thresh >>= 1;
> > > +
> > > + /*
> > > + * Clamp the delta to stay in the scheduler period range
> > > + * [-sysctl_sched_latency:sysctl_sched_latency]
> > > + */
> > > + latency_weight = clamp_t(long, latency_weight,
> > > + -1 * NICE_LATENCY_WEIGHT_MAX,
> > > + NICE_LATENCY_WEIGHT_MAX);
> > > +
> > > + return (thresh * latency_weight) >> NICE_LATENCY_SHIFT;
> > > +}
> > > +
> > > static unsigned long wakeup_gran(struct sched_entity *se)
> > > {
> > > unsigned long gran = sysctl_sched_wakeup_granularity;
> > > @@ -7008,6 +7059,10 @@ static int
> > > wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
> > > {
> > > s64 gran, vdiff = curr->vruntime - se->vruntime;
> > > + int latency_weight = se->latency_weight - curr->latency_weight;
> > > +
> > > + latency_weight = min(latency_weight, se->latency_weight);
> >
> > Let lable A=se->latency_weight, B=curr->latency_weight, C=A-B.
> >
> > 1 A>0 B>0
> > ::C=A-B<0, min(C,A)=C, latency decrease, C is the real diff value no matter
> > what A is. That means it can be very 'long'(-sched_latency) and vdiff is
> > more possible to be in <= 0 case and return -1.
> > ::C=A-B>0, min(C,A)=A, latency increase, but it is conservative. Limit to
> > A/1024*sched_latency.
> > When latecy is decreased, the negtive value added to vdiff is larger than the
> > positive value added to vdiff when latency is increased.
> >
> > 2 A>0 B<0
> > ::C=A-B>0 and C>A, min(C,A)=A, latency increase and it is conservative.
> >
> > 3 A<0 B<0
> > ::C=A-B>0, min(C,A)=A, latency increase but min(C,A)<0, I think if latency
> > increase means the value added to vdiff will be a positive value to increase
> > the chance to return 1. I would say it is max(C,A)=C
> > ::C=A-B<0, min(C,A)=A, latency decrease and the real negtive value.
> >
> > 4 A<0,B>0
> > ::C=A-B<0, min(C,A)=C, latency decrease and the real negtive value.
> >
> > Is there a reason that the value when latency increase and latency decrease
> > be treated differently. Latency increase value is limited to se's latency_weight
> > but latency decrease value can extend to -sched_latency or treat them the same.
> > Penalty latency decrease and conserve latency increase.
> >
> >
> > There is any value that this latency_weight can be considered to be a average.
> >
> > The delta value choose is ~%5 to 1024. %5*sched_latency=0.05*6ms=0.3ms.(no scale)
> > I do not think over that vdiff equation and others though.
>
> vruntime is the accumulated weight history scaled time.
> vdiff is the scaled diff time delta.
>
> latency_gran = [delta_weight|latency_weight]/1024*thresh.
> If also to scale this latency_gran to reflect the load weight
> distribution in sched_latency like:
>
> latency_gran*1024/load_weight=[delta_weight|latency_weight]/load_weight*thresh.

wakeup_gran already scales with se load_weight

>
> About [delta_weight|latency_weight]/load_weight. Because latency nice and prio nice
> range is the same. Can make a table to const this calculation.. But I do not
> make sure that it is valuable to make this change. Just some random inputs..
>
> > Thanks,
> > Tao
> > > + vdiff += wakeup_latency_gran(latency_weight);
> > >
> > > if (vdiff <= 0)
> > > return -1;
> > > @@ -7117,6 +7172,7 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
> > > return;
> > >
> > > update_curr(cfs_rq_of(se));
> > > +
> > > if (wakeup_preempt_entity(se, pse) == 1) {
> > > /*
> > > * Bias pick_next to pick the sched entity that is
> > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > > index 456ad2159eb1..dd92aa9c36f9 100644
> > > --- a/kernel/sched/sched.h
> > > +++ b/kernel/sched/sched.h
> > > @@ -122,6 +122,17 @@ extern void call_trace_sched_update_nr_running(struct rq *rq, int count);
> > > * Default tasks should be treated as a task with latency_nice = 0.
> > > */
> > > #define DEFAULT_LATENCY_NICE 0
> > > +#define DEFAULT_LATENCY_PRIO (DEFAULT_LATENCY_NICE + LATENCY_NICE_WIDTH/2)
> > > +
> > > +/*
> > > + * Convert user-nice values [ -20 ... 0 ... 19 ]
> > > + * to static latency [ 0..39 ],
> > > + * and back.
> > > + */
> > > +#define NICE_TO_LATENCY(nice) ((nice) + DEFAULT_LATENCY_PRIO)
> > > +#define LATENCY_TO_NICE(prio) ((prio) - DEFAULT_LATENCY_PRIO)
> > > +#define NICE_LATENCY_SHIFT (SCHED_FIXEDPOINT_SHIFT)
> > > +#define NICE_LATENCY_WEIGHT_MAX (1L << NICE_LATENCY_SHIFT)
> > >
> > > /*
> > > * Increase resolution of nice-level calculations for 64-bit architectures.
> > > @@ -2098,6 +2109,7 @@ static_assert(WF_TTWU == SD_BALANCE_WAKE);
> > >
> > > extern const int sched_prio_to_weight[40];
> > > extern const u32 sched_prio_to_wmult[40];
> > > +extern const int sched_latency_to_weight[40];
> > >
> > > /*
> > > * {de,en}queue flags:
> > > --
> > > 2.17.1
> > >

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