Re: [PATCH v3 2/2] sched/fair: Choose the CPU where short task is running during wake up

[Tianchen Ding]

Hi Chen.

On 2022/12/1 16:44, Chen Yu wrote:
> [Problem Statement]
> For a workload that is doing frequent context switches, the throughput
> scales well until the number of instances reaches a peak point. After
> that peak point, the throughput drops significantly if the number of
> instances continues to increase.
>
> The will-it-scale context_switch1 test case exposes the issue. The
> test platform has 112 CPUs per LLC domain. The will-it-scale launches
> 1, 8, 16 ... 112 instances respectively. Each instance is composed
> of 2 tasks, and each pair of tasks would do ping-pong scheduling via
> pipe_read() and pipe_write(). No task is bound to any CPU.
> It is found that, once the number of instances is higher than
> 56(112 tasks in total, every CPU has 1 task), the throughput
> drops accordingly if the instance number continues to increase:
>
>            ^
> throughput|
>            |                 X
>            |               X   X X
>            |             X         X X
>            |           X               X
>            |         X                   X
>            |       X
>            |     X
>            |   X
>            | X
>            |
>            +-----------------.------------------->
>                              56
>                                   number of instances
>
> [Symptom analysis]
>
> The performance downgrading was caused by a high system idle
> percentage(around 20% ~ 30%). The CPUs waste a lot of time in
> idle and do nothing. As a comparison, if set CPU affinity to
> these workloads and stops them from migrating among CPUs,
> the idle percentage drops to nearly 0%, and the throughput
> increases by about 300%. This indicates room for optimization.
>
> The reason for the high idle percentage is different before/after
> commit f3dd3f674555 ("sched: Remove the limitation of WF_ON_CPU
> on wakelist if wakee cpu is idle").
>
> [Before the commit]
> The bottleneck is the runqueue spinlock.
>
> nr_instance          rq lock percentage
> 1                    1.22%
> 8                    1.17%
> 16                   1.20%
> 24                   1.22%
> 32                   1.46%
> 40                   1.61%
> 48                   1.63%
> 56                   1.65%
> --------------------------
> 64                   3.77%      |
> 72                   5.90%      | increase
> 80                   7.95%      |
> 88                   9.98%      v
> 96                   11.81%
> 104                  13.54%
> 112                  15.13%
>
> And top 2 rq lock hot paths:
>
> (path1):
> raw_spin_rq_lock_nested.constprop.0;
> try_to_wake_up;
> default_wake_function;
> autoremove_wake_function;
> __wake_up_common;
> __wake_up_common_lock;
> __wake_up_sync_key;
> pipe_write;
> new_sync_write;
> vfs_write;
> ksys_write;
> __x64_sys_write;
> do_syscall_64;
> entry_SYSCALL_64_after_hwframe;write
>
> (path2):
> raw_spin_rq_lock_nested.constprop.0;
> __sched_text_start;
> schedule_idle;
> do_idle;
> cpu_startup_entry;
> start_secondary;
> secondary_startup_64_no_verify
>
> task A tries to wake up task B on CPU1, then task A grabs the
> runqueue lock of CPU1. If CPU1 is about to quit idle, it needs
> to grab its lock which has been taken by someone else. Then
> CPU1 takes more time to quit which hurts the performance.
>
> [After the commit]
> The cause is the race condition between select_task_rq() and
> the task enqueue.
>
> Suppose there are nr_cpus pairs of ping-pong scheduling
> tasks. For example, p0' and p0 are ping-pong scheduling,
> so do p1' <=> p1, and p2'<=> p2. None of these tasks are
> bound to any CPUs. The problem can be summarized as:
> more than 1 wakers are stacked on 1 CPU, which slows down
> waking up their wakees:
>
> CPU0					CPU1				CPU2
>
> p0'					p1' => idle			p2'
>
> try_to_wake_up(p0)							try_to_wake_up(p2);
> CPU1 = select_task_rq(p0);						CPU1 = select_task_rq(p2);
> ttwu_queue(p0, CPU1);							ttwu_queue(p2, CPU1);
>    __ttwu_queue_wakelist(p0, CPU1); =>	ttwu_list->p0
> 					quiting cpuidle_idle_call()
>
> 					ttwu_list->p2->p0	<=	  __ttwu_queue_wakelist(p2, CPU1);
>
>      WRITE_ONCE(CPU1->ttwu_pending, 1);					    WRITE_ONCE(CPU1->ttwu_pending, 1);
>
> p0' => idle
> 					sched_ttwu_pending()
> 					  enqueue_task(p2 and p0)
>
> 					idle => p2
>
> 					...
> 					p2 time slice expires
> 					...
> 									!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
> 								<===	!!! p2 delays the wake up of p0' !!!
> 									!!! causes long idle on CPU0     !!!
> 					p2 => p0			!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
> 					p0 wakes up p0'
>
> idle => p0'
>
>
>
> Since there are many waker/wakee pairs in the system, the chain reaction
> causes many CPUs to be victims. These idle CPUs wait for their waker to
> be scheduled.
>
> Actually Tiancheng has mentioned above issue here[2].

First I want to say that this issue (race condition between selecting idle cpu 
and enqueuing task) always exists before or after the commit f3dd3f674555. And I 
know this is a big issue so in [2] I only try to fix a small part of it. Of 
course I'm glad to see you trying solving this issue too :-)

There may be a bit wrong in your comment about the order. 
"WRITE_ONCE(CPU1->ttwu_pending, 1);" on CPU0 is earlier than CPU1 getting 
"ttwu_list->p0", right?

Thanks.