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Date: Sun, 1 May 2022 23:58:42 +0800
From: Tao Zhou <tao.zhou@...ux.dev>
To: Vincent Guittot <vincent.guittot@...aro.org>,
Tao Zhou <tao.zhou@...ux.dev>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
mgorman@...e.de, linux-kernel@...r.kernel.org, parth@...ux.ibm.com,
qais.yousef@....com, chris.hyser@...cle.com,
pkondeti@...eaurora.org, vschneid@...hat.com,
patrick.bellasi@...bug.net, David.Laight@...lab.com,
pjt@...gle.com, pavel@....cz, tj@...nel.org,
dhaval.giani@...cle.com, qperret@...gle.com,
tim.c.chen@...ux.intel.com
Subject: Re: [RFC 5/6] sched/fair: Take into account latency nice at wakeup
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@...aro.org>
> ---
> 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.
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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