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Date: Fri, 23 Oct 2020 09:19:05 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Don <joshdon@...gle.com>, g@...ez.programming.kicks-ass.net
Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Paolo Bonzini <pbonzini@...hat.com>,
Eric Dumazet <edumazet@...gle.com>,
linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
kvm@...r.kernel.org, Xi Wang <xii@...gle.com>
Subject: Re: [PATCH 1/3] sched: better handling for busy polling loops
On Thu, Oct 22, 2020 at 08:29:42PM -0700, Josh Don wrote:
> Busy polling loops in the kernel such as network socket poll and kvm
> halt polling have performance problems related to process scheduler load
> accounting.
AFAICT you're not actually fixing the load accounting issue at all.
> This change also disables preemption for the duration of the busy
> polling loop. This is important, as it ensures that if a polling thread
> decides to end its poll to relinquish cpu to another thread, the polling
> thread will actually exit the busy loop and potentially block. When it
> later becomes runnable, it will have the opportunity to find an idle cpu
> via wakeup cpu selection.
At the cost of inducing a sleep+wake cycle; which is mucho expensive. So
this could go either way. No data presented.
> +void prepare_to_busy_poll(void)
> +{
> + struct rq __maybe_unused *rq = this_rq();
> + unsigned long __maybe_unused flags;
> +
> + /* Preemption will be reenabled by end_busy_poll() */
> + preempt_disable();
> +
> +#ifdef CONFIG_SMP
> + raw_spin_lock_irqsave(&rq->lock, flags);
> + /* preemption disabled; only one thread can poll at a time */
> + WARN_ON_ONCE(rq->busy_polling);
> + rq->busy_polling++;
> + raw_spin_unlock_irqrestore(&rq->lock, flags);
> +#endif
Explain to me the purpose of that rq->lock usage.
> +}
> +EXPORT_SYMBOL(prepare_to_busy_poll);
_GPL
> +
> +int continue_busy_poll(void)
> +{
> + if (!single_task_running())
> + return 0;
Why? If there's more, we'll end up in the below condition anyway.
> +
> + /* Important that we check this, since preemption is disabled */
> + if (need_resched())
> + return 0;
> +
> + return 1;
> +}
> +EXPORT_SYMBOL(continue_busy_poll);
_GPL
> +
> +/*
> + * Restore any state modified by prepare_to_busy_poll(), including re-enabling
> + * preemption.
> + *
> + * @allow_resched: If true, this potentially calls schedule() as part of
> + * enabling preemption. A busy poll loop can use false in order to have an
> + * opportunity to block before rescheduling.
> + */
> +void end_busy_poll(bool allow_resched)
> +{
> +#ifdef CONFIG_SMP
> + struct rq *rq = this_rq();
> + unsigned long flags;
> +
> + raw_spin_lock_irqsave(&rq->lock, flags);
> + BUG_ON(!rq->busy_polling); /* not paired with prepare() */
> + rq->busy_polling--;
> + raw_spin_unlock_irqrestore(&rq->lock, flags);
> +#endif
Again, please explain this lock usage.
> +
> + /*
> + * preemption needs to be kept disabled between prepare_to_busy_poll()
> + * and end_busy_poll().
> + */
> + BUG_ON(preemptible());
> + if (allow_resched)
> + preempt_enable();
> + else
> + preempt_enable_no_resched();
NAK on @allow_resched
> +}
> +EXPORT_SYMBOL(end_busy_poll);
_GPL
> +
> #ifdef CONFIG_CGROUP_SCHED
> /* task_group_lock serializes the addition/removal of task groups */
> static DEFINE_SPINLOCK(task_group_lock);
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1a68a0536add..58e525c74cc6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5460,6 +5460,11 @@ static int sched_idle_cpu(int cpu)
> {
> return sched_idle_rq(cpu_rq(cpu));
> }
> +
> +static int sched_idle_or_polling_cpu(int cpu)
> +{
> + return sched_idle_cpu(cpu) || polling_cpu(cpu);
> +}
> #endif
>
> /*
> @@ -5880,6 +5885,7 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this
> u64 latest_idle_timestamp = 0;
> int least_loaded_cpu = this_cpu;
> int shallowest_idle_cpu = -1;
> + int found_polling = 0;
> int i;
>
> /* Check if we have any choice: */
> @@ -5914,10 +5920,14 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this
> shallowest_idle_cpu = i;
> }
> } else if (shallowest_idle_cpu == -1) {
> + int polling = polling_cpu(i);
> +
> load = cpu_load(cpu_rq(i));
> - if (load < min_load) {
> + if ((polling == found_polling && load < min_load) ||
> + (polling && !found_polling)) {
> min_load = load;
> least_loaded_cpu = i;
> + found_polling = polling;
> }
> }
> }
> @@ -6085,7 +6095,7 @@ static int select_idle_smt(struct task_struct *p, int target)
> for_each_cpu(cpu, cpu_smt_mask(target)) {
> if (!cpumask_test_cpu(cpu, p->cpus_ptr))
> continue;
> - if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
> + if (available_idle_cpu(cpu) || sched_idle_or_polling_cpu(cpu))
> return cpu;
> }
>
> @@ -6149,7 +6159,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> for_each_cpu_wrap(cpu, cpus, target) {
> if (!--nr)
> return -1;
> - if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
> + if (available_idle_cpu(cpu) || sched_idle_or_polling_cpu(cpu))
> break;
> }
>
> @@ -6179,7 +6189,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> for_each_cpu_wrap(cpu, cpus, target) {
> unsigned long cpu_cap = capacity_of(cpu);
>
> - if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
> + if (!available_idle_cpu(cpu) && !sched_idle_or_polling_cpu(cpu))
> continue;
> if (task_fits_capacity(p, cpu_cap))
> return cpu;
> @@ -6223,14 +6233,14 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> }
>
> symmetric:
> - if (available_idle_cpu(target) || sched_idle_cpu(target))
> + if (available_idle_cpu(target) || sched_idle_or_polling_cpu(target))
> return target;
>
> /*
> * If the previous CPU is cache affine and idle, don't be stupid:
> */
> if (prev != target && cpus_share_cache(prev, target) &&
> - (available_idle_cpu(prev) || sched_idle_cpu(prev)))
> + (available_idle_cpu(prev) || sched_idle_or_polling_cpu(prev)))
> return prev;
>
> /*
> @@ -6252,7 +6262,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> if (recent_used_cpu != prev &&
> recent_used_cpu != target &&
> cpus_share_cache(recent_used_cpu, target) &&
> - (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) &&
> + (available_idle_cpu(recent_used_cpu) ||
> + sched_idle_or_polling_cpu(recent_used_cpu)) &&
> cpumask_test_cpu(p->recent_used_cpu, p->cpus_ptr)) {
> /*
> * Replace recent_used_cpu with prev as it is a potential
None of this affects load-tracking
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 28709f6b0975..45de468d0ffb 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1003,6 +1003,8 @@ struct rq {
>
> /* This is used to determine avg_idle's max value */
> u64 max_idle_balance_cost;
> +
> + unsigned int busy_polling;
This is a good location, cache-wise, because?
> #endif /* CONFIG_SMP */
>
> #ifdef CONFIG_IRQ_TIME_ACCOUNTING
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