Re: [PATCH v12 5/8] sched/fair: Take into account latency priority at wakeup

From: shrikanth hegde
Date: Wed Mar 01 2023 - 14:29:36 EST



On 2/24/23 3:04 PM, Vincent Guittot wrote:
> Take into account the 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 1.378(+/- 1%) 1.337(+/- 1%) + 3%
> 4 1.393(+/- 3%) 1.312(+/- 3%) + 6%
> 8 1.308(+/- 2%) 1.279(+/- 1%) + 2%
> 16 1.347(+/- 1%) 1.317(+/- 1%) + 2%
>
> hackbench -p -l (2560 / group) -g group
> group
> 1 1.836(+/- 17%) 1.148(+/- 5%) +37%
> 4 1.586(+/- 6%) 1.109(+/- 8%) +30%
> 8 1.209(+/- 4%) 0.780(+/- 4%) +35%
> 16 0.805(+/- 5%) 0.728(+/- 4%) +10%
>
> By deacreasing the latency prio, we reduce the number of preemption at
> 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 19 29 17 18 29
> 1 43 299 7359 63 84 3422
> 4 56 449 14806 45 83 284
> 8 63 820 51123 63 83 283
> 16 64 1326 70684 41 157 26852
>
> 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 parameter 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: 64 62
> Avg Latencies: 1170 107
> Max Latencies: 88069 10417
> 50% latencies: 122 86
> 75% latencies: 614 91
> 85% latencies: 961 94
> 90% latencies: 1225 97
> 95% latencies: 6120 102
> 99% latencies: 18328 159
>
> With percentile details, we see the benefit of nice latency -20 as
> only 1% of the latencies are above 159us whereas the default latency
> has got 15% around ~1ms or above and 5% over the 6ms.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> ---
> include/linux/sched.h | 4 +++-
> include/linux/sched/prio.h | 9 +++++++++
> init/init_task.c | 2 +-
> kernel/sched/core.c | 19 ++++++++++++++-----
> kernel/sched/debug.c | 2 +-
> kernel/sched/fair.c | 32 +++++++++++++++++++++++++++-----
> kernel/sched/sched.h | 11 +++++++++++
> 7 files changed, 66 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 6c61bde49152..38decae3e156 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -568,6 +568,8 @@ struct sched_entity {
> /* cached value of my_q->h_nr_running */
> unsigned long runnable_weight;
> #endif
> + /* preemption offset in ns */
> + long latency_offset;
>
> #ifdef CONFIG_SMP
> /*
> @@ -784,7 +786,7 @@ 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/include/linux/sched/prio.h b/include/linux/sched/prio.h
> index bfcd7f1d1e11..be79503d86af 100644
> --- a/include/linux/sched/prio.h
> +++ b/include/linux/sched/prio.h
> @@ -59,5 +59,14 @@ static inline long rlimit_to_nice(long prio)
> * 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)
>
> #endif /* _LINUX_SCHED_PRIO_H */
> diff --git a/init/init_task.c b/init/init_task.c
> index 7dd71dd2d261..071deff8dbd1 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 = DEFAULT_LATENCY_NICE,
> + .latency_prio = DEFAULT_LATENCY_PRIO,
> .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 d327614c70b0..d5b7e237d79b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1285,6 +1285,11 @@ static void set_load_weight(struct task_struct *p, bool update_load)
> }
> }
>
> +static void set_latency_offset(struct task_struct *p)
> +{
> + p->se.latency_offset = calc_latency_offset(p->latency_prio);
> +}
> +
> #ifdef CONFIG_UCLAMP_TASK
> /*
> * Serializes updates of utilization clamp values
> @@ -4681,7 +4686,9 @@ 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);
> + set_latency_offset(p);
> +
> /*
> * We don't need the reset flag anymore after the fork. It has
> * fulfilled its duty:
> @@ -7449,8 +7456,10 @@ static void __setscheduler_params(struct task_struct *p,
> static void __setscheduler_latency(struct task_struct *p,
> const struct sched_attr *attr)
> {
> - if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE)
> - p->latency_nice = attr->sched_latency_nice;
> + if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE) {
> + p->latency_prio = NICE_TO_LATENCY(attr->sched_latency_nice);
> + set_latency_offset(p);
> + }
> }
>
> /*
> @@ -7635,7 +7644,7 @@ 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;
> @@ -8176,7 +8185,7 @@ 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
> /*
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 68be7a3e42a3..b3922184af91 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);

/proc/<pid>/sched should update if the latency values are updated
for the cgroup right? That doesn't seem to happen.

#cd /sys/fs/cgroup/cpu
# echo -20 > task1/cpu.latency.nice
# cat task1/cgroup.procs
1897
1998
1999
# cat /proc/1999/sched | grep latency
latency_prio : 20
# echo 0 > task1/cpu.latency.nice
# cat /proc/1999/sched | grep latency
latency_prio : 20
# echo 19 > task1/cpu.latency.nice
# cat /proc/1999/sched | grep latency
latency_prio : 20


> 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 81bef11eb660..414b6243208b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4877,6 +4877,8 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> update_idle_cfs_rq_clock_pelt(cfs_rq);
> }
>
> +static long wakeup_latency_gran(struct sched_entity *curr, struct sched_entity *se);
> +
> /*
> * Preempt the current task with a newly woken task if needed:
> */
> @@ -4885,7 +4887,7 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> {
> unsigned long ideal_runtime, delta_exec;
> struct sched_entity *se;
> - s64 delta;
> + s64 delta, offset;
>
> /*
> * When many tasks blow up the sched_period; it is possible that
> @@ -4916,10 +4918,12 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> se = __pick_first_entity(cfs_rq);
> delta = curr->vruntime - se->vruntime;
>
> - if (delta < 0)
> + offset = wakeup_latency_gran(curr, se);
> + if (delta < offset)
> return;
>
> - if (delta > ideal_runtime)
> + if ((delta > ideal_runtime) ||
> + (delta > get_latency_max()))
> resched_curr(rq_of(cfs_rq));
> }
>
> @@ -7662,6 +7666,23 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> }
> #endif /* CONFIG_SMP */
>
> +static long wakeup_latency_gran(struct sched_entity *curr, struct sched_entity *se)
> +{
> + long latency_offset = se->latency_offset;
> +
> + /*
> + * A negative latency offset means that the sched_entity has latency
> + * requirement that needs to be evaluated versus other entity.
> + * Otherwise, use the latency weight to evaluate how much scheduling
> + * delay is acceptable by se.
> + */
> + if ((latency_offset < 0) || (curr->latency_offset < 0))
> + latency_offset -= curr->latency_offset;
> + latency_offset = min_t(long, latency_offset, get_latency_max());
> +
> + return latency_offset;
> +}
> +
> static unsigned long wakeup_gran(struct sched_entity *se)
> {
> unsigned long gran = sysctl_sched_wakeup_granularity;
> @@ -7700,11 +7721,12 @@ static int
> wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
> {
> s64 gran, vdiff = curr->vruntime - se->vruntime;
> + s64 offset = wakeup_latency_gran(curr, se);
>
> - if (vdiff <= 0)
> + if (vdiff < offset)
> return -1;
>
> - gran = wakeup_gran(se);
> + gran = offset + wakeup_gran(se);
>
> /*
> * At wake up, the vruntime of a task is capped to not be older than
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 51ba0af7fb27..3f42f86105d4 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2494,6 +2494,17 @@ static inline unsigned long get_sleep_latency(bool idle)
> return thresh;
> }
>
> +/*
> + * Calculate the latency offset for a priority level.
> + * We use a linear mapping of the priority in the range:
> + * [-sysctl_sched_latency:sysctl_sched_latency]
> + */
> +static inline long calc_latency_offset(int prio)
> +{
> + return (long)get_sleep_latency(false) * LATENCY_TO_NICE(prio) /
> + (LATENCY_NICE_WIDTH/2);
> +}
> +
> static inline unsigned long get_latency_max(void)
> {
> unsigned long thresh = get_sleep_latency(false);