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 >
