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Subject[PATCH RESEND v4 2/2] sched: Remove the limitation of WF_ON_CPU on wakelist if wakee cpu is idle
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Wakelist can help avoid cache bouncing and offload the overhead of waker
cpu. So far, using wakelist within the same llc only happens on
WF_ON_CPU, and this limitation could be removed to further improve
wakeup performance.

The commit 518cd6234178 ("sched: Only queue remote wakeups when
crossing cache boundaries") disabled queuing tasks on wakelist when
the cpus share llc. This is because, at that time, the scheduler must
send IPIs to do ttwu_queue_wakelist. Nowadays, ttwu_queue_wakelist also
supports TIF_POLLING, so this is not a problem now when the wakee cpu is
in idle polling.

Benefits:
Queuing the task on idle cpu can help improving performance on waker cpu
and utilization on wakee cpu, and further improve locality because
the wakee cpu can handle its own rq. This patch helps improving rt on
our real java workloads where wakeup happens frequently.

Consider the normal condition (CPU0 and CPU1 share same llc)
Before this patch:

CPU0 CPU1

select_task_rq() idle
rq_lock(CPU1->rq)
enqueue_task(CPU1->rq)
notify CPU1 (by sending IPI or CPU1 polling)

resched()

After this patch:

CPU0 CPU1

select_task_rq() idle
add to wakelist of CPU1
notify CPU1 (by sending IPI or CPU1 polling)

rq_lock(CPU1->rq)
enqueue_task(CPU1->rq)
resched()

We see CPU0 can finish its work earlier. It only needs to put task to
wakelist and return.
While CPU1 is idle, so let itself handle its own runqueue data.

This patch brings no difference about IPI.
This patch only takes effect when the wakee cpu is:
1) idle polling
2) idle not polling

For 1), there will be no IPI with or without this patch.

For 2), there will always be an IPI before or after this patch.
Before this patch: waker cpu will enqueue task and check preempt. Since
"idle" will be sure to be preempted, waker cpu must send a resched IPI.
After this patch: waker cpu will put the task to the wakelist of wakee
cpu, and send an IPI.

Benchmark:
We've tested schbench, unixbench, and hachbench on both x86 and arm64.

On x86 (Intel Xeon Platinum 8269CY):
schbench -m 2 -t 8

Latency percentiles (usec) before after
50.0000th: 8 6
75.0000th: 10 7
90.0000th: 11 8
95.0000th: 12 8
*99.0000th: 13 10
99.5000th: 15 11
99.9000th: 18 14

Unixbench with full threads (104)
before after
Dhrystone 2 using register variables 3011862938 3009935994 -0.06%
Double-Precision Whetstone 617119.3 617298.5 0.03%
Execl Throughput 27667.3 27627.3 -0.14%
File Copy 1024 bufsize 2000 maxblocks 785871.4 784906.2 -0.12%
File Copy 256 bufsize 500 maxblocks 210113.6 212635.4 1.20%
File Copy 4096 bufsize 8000 maxblocks 2328862.2 2320529.1 -0.36%
Pipe Throughput 145535622.8 145323033.2 -0.15%
Pipe-based Context Switching 3221686.4 3583975.4 11.25%
Process Creation 101347.1 103345.4 1.97%
Shell Scripts (1 concurrent) 120193.5 123977.8 3.15%
Shell Scripts (8 concurrent) 17233.4 17138.4 -0.55%
System Call Overhead 5300604.8 5312213.6 0.22%

hackbench -g 1 -l 100000
before after
Time 3.246 2.251

On arm64 (Ampere Altra):
schbench -m 2 -t 8

Latency percentiles (usec) before after
50.0000th: 14 10
75.0000th: 19 14
90.0000th: 22 16
95.0000th: 23 16
*99.0000th: 24 17
99.5000th: 24 17
99.9000th: 28 25

Unixbench with full threads (80)
before after
Dhrystone 2 using register variables 3536194249 3537019613 0.02%
Double-Precision Whetstone 629383.6 629431.6 0.01%
Execl Throughput 65920.5 65846.2 -0.11%
File Copy 1024 bufsize 2000 maxblocks 1063722.8 1064026.8 0.03%
File Copy 256 bufsize 500 maxblocks 322684.5 318724.5 -1.23%
File Copy 4096 bufsize 8000 maxblocks 2348285.3 2328804.8 -0.83%
Pipe Throughput 133542875.3 131619389.8 -1.44%
Pipe-based Context Switching 3215356.1 3576945.1 11.25%
Process Creation 108520.5 120184.6 10.75%
Shell Scripts (1 concurrent) 122636.3 121888 -0.61%
Shell Scripts (8 concurrent) 17462.1 17381.4 -0.46%
System Call Overhead 4429998.9 4435006.7 0.11%

hackbench -g 1 -l 100000
before after
Time 4.217 2.916

Our patch has improvement on schbench, hackbench
and Pipe-based Context Switching of unixbench
when there exists idle cpus,
and no obvious regression on other tests of unixbench.
This can help improve rt in scenes where wakeup happens frequently.

Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
---
kernel/sched/core.c | 26 ++++++++++++++------------
kernel/sched/sched.h | 1 -
2 files changed, 14 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a4bdb2b95976..4d2bb78276bc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3808,7 +3808,7 @@ bool cpus_share_cache(int this_cpu, int that_cpu)
return per_cpu(sd_llc_id, this_cpu) == per_cpu(sd_llc_id, that_cpu);
}

-static inline bool ttwu_queue_cond(int cpu, int wake_flags)
+static inline bool ttwu_queue_cond(int cpu)
{
/*
* Do not complicate things with the async wake_list while the CPU is
@@ -3824,17 +3824,21 @@ static inline bool ttwu_queue_cond(int cpu, int wake_flags)
if (!cpus_share_cache(smp_processor_id(), cpu))
return true;

+ if (cpu == smp_processor_id())
+ return false;
+
/*
- * If the task is descheduling and the only running task on the
- * CPU then use the wakelist to offload the task activation to
- * the soon-to-be-idle CPU as the current CPU is likely busy.
- * nr_running is checked to avoid unnecessary task stacking.
+ * If the wakee cpu is idle, or the task is descheduling and the
+ * only running task on the CPU, then use the wakelist to offload
+ * the task activation to the idle (or soon-to-be-idle) CPU as
+ * the current CPU is likely busy. nr_running is checked to
+ * avoid unnecessary task stacking.
*
* Note that we can only get here with (wakee) p->on_rq=0,
* p->on_cpu can be whatever, we've done the dequeue, so
* the wakee has been accounted out of ->nr_running.
*/
- if ((wake_flags & WF_ON_CPU) && !cpu_rq(cpu)->nr_running)
+ if (!cpu_rq(cpu)->nr_running)
return true;

return false;
@@ -3842,10 +3846,7 @@ static inline bool ttwu_queue_cond(int cpu, int wake_flags)

static bool ttwu_queue_wakelist(struct task_struct *p, int cpu, int wake_flags)
{
- if (sched_feat(TTWU_QUEUE) && ttwu_queue_cond(cpu, wake_flags)) {
- if (WARN_ON_ONCE(cpu == smp_processor_id()))
- return false;
-
+ if (sched_feat(TTWU_QUEUE) && ttwu_queue_cond(cpu)) {
sched_clock_cpu(cpu); /* Sync clocks across CPUs */
__ttwu_queue_wakelist(p, cpu, wake_flags);
return true;
@@ -4167,7 +4168,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
* scheduling.
*/
if (smp_load_acquire(&p->on_cpu) &&
- ttwu_queue_wakelist(p, task_cpu(p), wake_flags | WF_ON_CPU))
+ ttwu_queue_wakelist(p, task_cpu(p), wake_flags))
goto unlock;

/*
@@ -4789,7 +4790,8 @@ static inline void prepare_task(struct task_struct *next)
* Claim the task as running, we do this before switching to it
* such that any running task will have this set.
*
- * See the ttwu() WF_ON_CPU case and its ordering comment.
+ * See the smp_load_acquire(&p->on_cpu) case in ttwu() and
+ * its ordering comment.
*/
WRITE_ONCE(next->on_cpu, 1);
#endif
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 01259611beb9..1e34bb4527fd 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2039,7 +2039,6 @@ static inline int task_on_rq_migrating(struct task_struct *p)

#define WF_SYNC 0x10 /* Waker goes to sleep after wakeup */
#define WF_MIGRATED 0x20 /* Internal use, task got migrated */
-#define WF_ON_CPU 0x40 /* Wakee is on_cpu */

#ifdef CONFIG_SMP
static_assert(WF_EXEC == SD_BALANCE_EXEC);
--
2.27.0
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