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

[Shrikanth Hegde <sshegde@linux.vnet.ibm.com>]

On 3/2/23 1:13 PM, Vincent Guittot wrote:
> On Wed, 1 Mar 2023 at 20:29, shrikanth hegde <sshegde@linux.vnet.ibm.com> wrote:
>>
>>
>> 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@linaro.org>
>>> Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
>>> ---
>>>  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.
> No It's not. The cgroup latency_nice value applies the the
> sched_entity of the group in which the task are scheduled


If this isnt per task, what is the use case of printing latency_prio 
/proc/<pid>/sched?

>
>> #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);