Messages in this thread Patch in this message | | | Date | Thu, 21 May 2020 13:41:32 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 00/13] Reconcile NUMA balancing decisions with the load balancer v6 |
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On Thu, May 21, 2020 at 11:38:16AM +0100, Mel Gorman wrote: > IIUC, this patch front-loads as much work as possible before checking if > the task is on_rq and then the waker/wakee shares a cache, queue task on > the wake_list and otherwise do a direct wakeup. > > The advantage is that spinning is avoided on p->on_rq when p does not > share a cache. The disadvantage is that it may result in tasks being > stacked but this should only happen when the domain is overloaded and > select_task_eq() is unlikely to find an idle CPU. The load balancer would > soon correct the situation anyway. > > In terms of netperf for my testing, the benefit is marginal because the > wakeups are primarily between tasks that share cache. It does trigger as > perf indicates that some time is spent in ttwu_queue_remote with this > patch, it's just that the overall time spent spinning on p->on_rq is > very similar. I'm still waiting on other workloads to complete to see > what the impact is.
So it might make sense to play with the exact conditions under which we'll attempt this remote queue, if we see a large 'local' p->on_cpu spin time, it might make sense to attempt the queue even in this case.
We could for example change it to:
if (REAC_ONCE(p->on_cpu) && ttwu_queue_remote(p, cpu, wake_flags | WF_ON_CPU)) goto unlock;
and then use that in ttwu_queue_remote() to differentiate between these two cases.
Anyway, if it's a wash (atomic op vs spinning) then it's probably not worth it.
Another optimization might be to forgo the IPI entirely in this case and instead stick a sched_ttwu_pending() at the end of __schedule() or something like that.
> However, intuitively at least, it makes sense to avoid spinning on p->on_rq > when it's unnecessary and the other changes appear to be safe. Even if > wake_list should be used in some cases for local wakeups, it would make > sense to put that on top of this patch. Do you want to slap a changelog > around this and update the comments or do you want me to do it? I should > have more results in a few hours even if they are limited to one machine > but ideally Rik would test his workload too.
I've written you a Changelog, but please carry it in your set to evaluate if it's actually worth it.
--- Subject: sched: Optimize ttwu() spinning on p->on_cpu From: Peter Zijlstra <peterz@infradead.org> Date: Fri, 15 May 2020 16:24:44 +0200
Both Rik and Mel reported seeing ttwu() spend significant time on:
smp_cond_load_acquire(&p->on_cpu, !VAL);
Attempt to avoid this by queueing the wakeup on the CPU that own's the p->on_cpu value. This will then allow the ttwu() to complete without further waiting.
Since we run schedule() with interrupts disabled, the IPI is guaranteed to happen after p->on_cpu is cleared, this is what makes it safe to queue early.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- kernel/sched/core.c | 45 ++++++++++++++++++++++++--------------------- 1 file changed, 24 insertions(+), 21 deletions(-)
--- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2312,7 +2312,7 @@ static void wake_csd_func(void *info) sched_ttwu_pending(); } -static void ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags) +static void __ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags) { struct rq *rq = cpu_rq(cpu); @@ -2354,6 +2354,17 @@ bool cpus_share_cache(int this_cpu, int { return per_cpu(sd_llc_id, this_cpu) == per_cpu(sd_llc_id, that_cpu); } + +static bool ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags) +{ + if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) { + sched_clock_cpu(cpu); /* Sync clocks across CPUs */ + __ttwu_queue_remote(p, cpu, wake_flags); + return true; + } + + return false; +} #endif /* CONFIG_SMP */ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags) @@ -2362,11 +2373,8 @@ static void ttwu_queue(struct task_struc struct rq_flags rf; #if defined(CONFIG_SMP) - if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) { - sched_clock_cpu(cpu); /* Sync clocks across CPUs */ - ttwu_queue_remote(p, cpu, wake_flags); + if (ttwu_queue_remote(p, cpu, wake_flags)) return; - } #endif rq_lock(rq, &rf); @@ -2550,7 +2558,15 @@ try_to_wake_up(struct task_struct *p, un if (p->on_rq && ttwu_remote(p, wake_flags)) goto unlock; + if (p->in_iowait) { + delayacct_blkio_end(p); + atomic_dec(&task_rq(p)->nr_iowait); + } + #ifdef CONFIG_SMP + p->sched_contributes_to_load = !!task_contributes_to_load(p); + p->state = TASK_WAKING; + /* * Ensure we load p->on_cpu _after_ p->on_rq, otherwise it would be * possible to, falsely, observe p->on_cpu == 0. @@ -2581,15 +2597,10 @@ try_to_wake_up(struct task_struct *p, un * This ensures that tasks getting woken will be fully ordered against * their previous state and preserve Program Order. */ - smp_cond_load_acquire(&p->on_cpu, !VAL); - - p->sched_contributes_to_load = !!task_contributes_to_load(p); - p->state = TASK_WAKING; + if (READ_ONCE(p->on_cpu) && ttwu_queue_remote(p, cpu, wake_flags)) + goto unlock; - if (p->in_iowait) { - delayacct_blkio_end(p); - atomic_dec(&task_rq(p)->nr_iowait); - } + smp_cond_load_acquire(&p->on_cpu, !VAL); cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags); if (task_cpu(p) != cpu) { @@ -2597,14 +2608,6 @@ try_to_wake_up(struct task_struct *p, un psi_ttwu_dequeue(p); set_task_cpu(p, cpu); } - -#else /* CONFIG_SMP */ - - if (p->in_iowait) { - delayacct_blkio_end(p); - atomic_dec(&task_rq(p)->nr_iowait); - } - #endif /* CONFIG_SMP */ ttwu_queue(p, cpu, wake_flags);
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