Re: [PATCH v2] sched/fair: Reschedule the cfs_rq when current is ineligible

[Chen Yu]

On 2024-05-29 at 22:18:06 +0800, Chunxin Zang wrote:
> I found that some tasks have been running for a long enough time and
> have become illegal, but they are still not releasing the CPU. This
> will increase the scheduling delay of other processes. Therefore, I
> tried checking the current process in wakeup_preempt and entity_tick,
> and if it is illegal, reschedule that cfs queue.
> 
> When RUN_TO_PARITY is enabled, its behavior essentially remains
> consistent with the original process. When NO_RUN_TO_PARITY is enabled,
> some additional preemptions will be introduced, but not too many.
> 
> I have pasted some test results below.
> I isolated four cores for testing and ran hackbench in the background,
> and observed the test results of cyclictest.
> 
> hackbench -g 4 -l 100000000 &
> cyclictest --mlockall -D 5m -q
> 
>                                  EEVDF      PATCH  EEVDF-NO_PARITY  PATCH-NO_PARITY
> 
>                 # Min Latencies: 00006      00006      00006      00006
>   LNICE(-19)    # Avg Latencies: 00191      00133      00089      00066
>                 # Max Latencies: 15442      08466      14133      07713
> 
>                 # Min Latencies: 00006      00010      00006      00006
>   LNICE(0)      # Avg Latencies: 00466      00326      00289      00257
>                 # Max Latencies: 38917      13945      32665      17710
> 
>                 # Min Latencies: 00019      00053      00010      00013
>   LNICE(19)     # Avg Latencies: 37151      25852      18293      23035
>                 # Max Latencies: 2688299    4643635    426196     425708
> 
> I captured and compared the number of preempt occurrences in wakeup_preempt
> to see if it introduced any additional overhead.
> 
> Similarly, hackbench is used to stress the utilization of four cores to
> 100%, and the method for capturing the number of PREEMPT occurrences is
> referenced from [1].
> 
> schedstats                          EEVDF       PATCH   EEVDF-NO_PARITY  PATCH-NO_PARITY  CFS(6.5)
> .stats.check_preempt_count          5053054     5045388    5018589    5029585
> .stats.patch_preempt_count          -------     0020495    -------    0700670    -------
> .stats.need_preempt_count           0570520     0458947    3380513    3116966    1140821
> 
> From the above test results, there is a slight increase in the number of
> preempt occurrences in wakeup_preempt. However, the results vary with each
> test, and sometimes the difference is not that significant.
> 
> [1]: https://lore.kernel.org/all/20230816134059.GC982867@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#m52057282ceb6203318be1ce9f835363de3bef5cb
> 
> Signed-off-by: Chunxin Zang <zangchunxin@xxxxxxxxxxx>
> Reviewed-by: Chen Yang <yangchen11@xxxxxxxxxxx>
> 
> ------
> Changes in v2:
> - Make the logic that determines the current process as ineligible and
>   triggers preemption effective only when NO_RUN_TO_PARITY is enabled.
> - Update the commit message
> ---
>  kernel/sched/fair.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 03be0d1330a6..fa2c512139e5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -745,6 +745,17 @@ int entity_eligible(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  	return vruntime_eligible(cfs_rq, se->vruntime);
>  }
>  
> +static bool check_entity_need_preempt(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +{
> +	if (sched_feat(RUN_TO_PARITY) && se->vlag != se->deadline)
> +		return true;

If I understand correctly, here it intends to check if the current se
has consumed its 1st slice after been picked at set_next_entity(), and if yes do a reschedule.
check_entity_need_preempt() is added at the end of entity_tick(), which could overwrite
the police to reschedule current: (entity_tick()->update_curr()->update_deadline()), only there
are more than 1 runnable tasks will the current be preempted, even if it has expired the 1st
requested slice.

> +
> +	if (!sched_feat(RUN_TO_PARITY) && !entity_eligible(cfs_rq, se))
> +		return true;
> +
> +	return false;
> +}
> +
>  static u64 __update_min_vruntime(struct cfs_rq *cfs_rq, u64 vruntime)
>  {
>  	u64 min_vruntime = cfs_rq->min_vruntime;
> @@ -5523,6 +5534,9 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
>  			hrtimer_active(&rq_of(cfs_rq)->hrtick_timer))
>  		return;
>  #endif
> +
> +	if (check_entity_need_preempt(cfs_rq, curr))
> +		resched_curr(rq_of(cfs_rq));
>  }
>  
>  
> @@ -8343,6 +8357,9 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>  	cfs_rq = cfs_rq_of(se);
>  	update_curr(cfs_rq);
>  
> +	if (check_entity_need_preempt(cfs_rq, se))
> +		goto preempt;
> +

As we changes the preemption policy for current in two places, the tick preemption and wakeup preemption,
do you have statistics that shows which one brings the most benefit?

thanks,
Chenyu