Re: [PATCH v2] sched/core: Preempt current task in favour of bound kthread

[Vincent Guittot <vincent.guittot@linaro.org>]

On Tue, 10 Dec 2019 at 06:43, Srikar Dronamraju
<srikar@linux.vnet.ibm.com> wrote:
>
> A running task can wake-up a per CPU bound kthread on the same CPU.
> If the current running task doesn't yield the CPU before the next load
> balance operation, the scheduler would detect load imbalance and try to
> balance the load. However this load balance would fail as the waiting
> task is CPU bound, while the running task cannot be moved by the regular
> load balancer. Finally the active load balancer would kick in and move
> the task to a different CPU/Core. Moving the task to a different
> CPU/core can lead to loss in cache affinity leading to poor performance.
>
> This is more prone to happen if the current running task is CPU
> intensive and the sched_wake_up_granularity is set to larger value.
> When the sched_wake_up_granularity was relatively small, it was observed
> that the bound thread would complete before the load balancer would have
> chosen to move the cache hot task to a different CPU.
>
> To deal with this situation, the current running task would yield to a
> per CPU bound kthread, provided kthread is not CPU intensive.
>
> /pboffline/hwcct_prg_old/lib/fsperf -t overwrite --noclean -f 5g -b 4k /pboffline
>
> (With sched_wake_up_granularity set to 15ms)

So you increase sched_wake_up_granularity to a high level to ensure
that current is no preempted by waking thread but then you add a way
to finally preempt it which is somewhat weird IMO

Have you tried to increase the priority of workqueue thread  (decrease
nice priority) ? This is the right way to reduce the impact of the
sched_wake_up_granularity on the wakeup of your specific kthread.
Because what you want at the end is keeping a low wakeup granularity
for these io workqueues

>
> Performance counter stats for 'system wide' (5 runs):
> event                                        v5.4                               v5.4 + patch(v2)
> probe:active_load_balance_cpu_stop       1,919  ( +-  2.89% )                   5  ( +- 12.56% )
> sched:sched_waking                     441,535  ( +-  0.17% )             901,174  ( +-  0.25% )
> sched:sched_wakeup                     441,533  ( +-  0.17% )             901,172  ( +-  0.25% )
> sched:sched_wakeup_new                   2,436  ( +-  8.08% )                 525  ( +-  2.57% )
> sched:sched_switch                     797,007  ( +-  0.26% )           1,458,463  ( +-  0.24% )
> sched:sched_migrate_task                20,998  ( +-  1.04% )               2,279  ( +-  3.47% )
> sched:sched_process_free                 2,436  ( +-  7.90% )                 527  ( +-  2.30% )
> sched:sched_process_exit                 2,451  ( +-  7.85% )                 542  ( +-  2.24% )
> sched:sched_wait_task                        7  ( +- 21.20% )                   1  ( +- 77.46% )
> sched:sched_process_wait                 3,951  ( +-  9.14% )                 816  ( +-  3.52% )
> sched:sched_process_fork                 2,435  ( +-  8.09% )                 524  ( +-  2.58% )
> sched:sched_process_exec                 1,023  ( +- 12.21% )                 198  ( +-  3.23% )
> sched:sched_wake_idle_without_ipi      187,794  ( +-  1.14% )             348,565  ( +-  0.34% )
>
> Elasped time in seconds          289.43 +- 1.42 ( +-  0.49% )    72.6013 +- 0.0417 ( +-  0.06% )
> Throughput results
>
> v5.4
> Trigger time:................... 0.842679 s   (Throughput:     6075.86 MB/s)
> Asynchronous submit time:.......   1.0184 s   (Throughput:     5027.49 MB/s)
> Synchronous submit time:........        0 s   (Throughput:           0 MB/s)
> I/O time:.......................   263.17 s   (Throughput:      19.455 MB/s)
> Ratio trigger time to I/O time:.0.00320202
>
> v5.4 + patch(v2)
> Trigger time:................... 0.853973 s   (Throughput:      5995.5 MB/s)
> Asynchronous submit time:....... 0.768092 s   (Throughput:     6665.86 MB/s)
> Synchronous submit time:........        0 s   (Throughput:           0 MB/s)
> I/O time:.......................  44.0267 s   (Throughput:     116.292 MB/s)
> Ratio trigger time to I/O time:.0.0193966
>
> (With sched_wake_up_granularity set to 4ms)
>
> Performance counter stats for 'system wide' (5 runs):
> event                                         v5.4                              v5.4 + patch(v2)
> probe:active_load_balance_cpu_stop               6  ( +-  6.03% )                   5  ( +- 23.20% )
> sched:sched_waking                         899,880  ( +-  0.38% )             899,737  ( +-  0.41% )
> sched:sched_wakeup                         899,878  ( +-  0.38% )             899,736  ( +-  0.41% )
> sched:sched_wakeup_new                         622  ( +- 11.95% )                 499  ( +-  1.08% )
> sched:sched_switch                       1,458,214  ( +-  0.40% )           1,451,374  ( +-  0.32% )
> sched:sched_migrate_task                     3,120  ( +- 10.00% )               2,500  ( +- 10.86% )
> sched:sched_process_free                       608  ( +- 12.18% )                 484  ( +-  1.19% )
> sched:sched_process_exit                       623  ( +- 11.91% )                 499  ( +-  1.15% )
> sched:sched_wait_task                            1  ( +- 31.18% )                   1  ( +- 31.18% )
> sched:sched_process_wait                       998  ( +- 13.22% )                 765  ( +-  0.16% )
> sched:sched_process_fork                       622  ( +- 11.95% )                 498  ( +-  1.08% )
> sched:sched_process_exec                       242  ( +- 13.81% )                 183  ( +-  0.48% )
> sched:sched_wake_idle_without_ipi          349,165  ( +-  0.35% )             347,773  ( +-  0.43% )
>
> Elasped time in seconds           72.8560 +- 0.0768 ( +-  0.11% )     72.4327 +- 0.0797 ( +-  0.11% )
>
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> ---
> Changelog:
> v1 : http://lore.kernel.org/lkml/20191209165122.GA27229@linux.vnet.ibm.com
> v1->v2: Pass the the right params to try_to_wake_up as correctly pointed out
> by Dave Chinner
>
>
>  kernel/sched/core.c  |  7 ++++++-
>  kernel/sched/fair.c  | 23 ++++++++++++++++++++++-
>  kernel/sched/sched.h |  3 ++-
>  3 files changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 44123b4d14e8..82126cbf62cd 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2664,7 +2664,12 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>   */
>  int wake_up_process(struct task_struct *p)
>  {
> -       return try_to_wake_up(p, TASK_NORMAL, 0);
> +       int wake_flags = 0;
> +
> +       if (is_per_cpu_kthread(p))
> +               wake_flags = WF_KTHREAD;
> +
> +       return try_to_wake_up(p, TASK_NORMAL, wake_flags);
>  }
>  EXPORT_SYMBOL(wake_up_process);
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69a81a5709ff..36486f71e59f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6660,6 +6660,27 @@ static void set_skip_buddy(struct sched_entity *se)
>                 cfs_rq_of(se)->skip = se;
>  }
>
> +static int kthread_wakeup_preempt(struct rq *rq, struct task_struct *p, int wake_flags)
> +{
> +       struct task_struct *curr = rq->curr;
> +       struct cfs_rq *cfs_rq = task_cfs_rq(curr);
> +
> +       if (!(wake_flags & WF_KTHREAD))
> +               return 0;
> +
> +       if (p->nr_cpus_allowed != 1 || curr->nr_cpus_allowed == 1)
> +               return 0;
> +
> +       if (cfs_rq->nr_running > 2)
> +               return 0;
> +
> +       /*
> +        * Don't preempt, if the waking kthread is more CPU intensive than
> +        * the current thread.
> +        */
> +       return p->nvcsw * curr->nivcsw >= p->nivcsw * curr->nvcsw;
> +}
> +
>  /*
>   * Preempt the current task with a newly woken task if needed:
>   */
> @@ -6716,7 +6737,7 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
>         find_matching_se(&se, &pse);
>         update_curr(cfs_rq_of(se));
>         BUG_ON(!pse);
> -       if (wakeup_preempt_entity(se, pse) == 1) {
> +       if (wakeup_preempt_entity(se, pse) == 1 || kthread_wakeup_preempt(rq, p, wake_flags)) {
>                 /*
>                  * Bias pick_next to pick the sched entity that is
>                  * triggering this preemption.
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index c8870c5bd7df..23d4284ad1e3 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1643,7 +1643,8 @@ static inline int task_on_rq_migrating(struct task_struct *p)
>   */
>  #define WF_SYNC                        0x01            /* Waker goes to sleep after wakeup */
>  #define WF_FORK                        0x02            /* Child wakeup after fork */
> -#define WF_MIGRATED            0x4             /* Internal use, task got migrated */
> +#define WF_MIGRATED            0x04            /* Internal use, task got migrated */
> +#define WF_KTHREAD             0x08            /* Per CPU Kthread*/
>
>  /*
>   * To aid in avoiding the subversion of "niceness" due to uneven distribution
> --
> 2.18.1
>