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

From: Vincent Guittot
Date: Tue Dec 10 2019 - 04:44:03 EST

On Tue, 10 Dec 2019 at 06:43, Srikar Dronamraju
<srikar@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> ---
> Changelog:
> v1 : http://lore.kernel.org/lkml/20191209165122.GA27229@xxxxxxxxxxxxxxxxxx
> 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
>