Messages in this thread |  | | Date | Mon, 5 Dec 2022 10:36:25 +0800 | Subject | Re: [PATCH v3 2/2] sched/fair: Choose the CPU where short task is running during wake up | From | Tianchen Ding <> |
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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.
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