Commit 4cfafd30 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Thomas Gleixner
Browse files

sched,perf: Fix periodic timers 



In the below two commits (see Fixes) we have periodic timers that can
stop themselves when they're no longer required, but need to be
(re)-started when their idle condition changes.

Further complications is that we want the timer handler to always do
the forward such that it will always correctly deal with the overruns,
and we do not want to race such that the handler has already decided
to stop, but the (external) restart sees the timer still active and we
end up with a 'lost' timer.

The problem with the current code is that the re-start can come before
the callback does the forward, at which point the forward from the
callback will WARN about forwarding an enqueued timer.

Now, conceptually its easy to detect if you're before or after the fwd
by comparing the expiration time against the current time. Of course,
that's expensive (and racy) because we don't have the current time.

Alternatively one could cache this state inside the timer, but then
everybody pays the overhead of maintaining this extra state, and that
is undesired.

The only other option that I could see is the external timer_active
variable, which I tried to kill before. I would love a nicer interface
for this seemingly simple 'problem' but alas.

Fixes: 272325c4 ("perf: Fix mux_interval hrtimer wreckage")
Fixes: 77a4d1a1 ("sched: Cleanup bandwidth timers")
Cc: pjt@google.com
Cc: tglx@linutronix.de
Cc: klamm@yandex-team.ru
Cc: mingo@kernel.org
Cc: bsegall@google.com
Cc: hpa@zytor.com
Cc: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20150514102311.GX21418@twins.programming.kicks-ass.net
parent c78c8818

include/linux/perf_event.h

+4 −0
Original line number Diff line number Diff line
@@ -566,8 +566,12 @@ struct perf_cpu_context { 
	struct perf_event_context	*task_ctx;

	int				active_oncpu;

	int				exclusive;



	raw_spinlock_t			hrtimer_lock;

	struct hrtimer			hrtimer;

	ktime_t				hrtimer_interval;

	unsigned int			hrtimer_active;



	struct pmu			*unique_pmu;

	struct perf_cgroup		*cgrp;

};

kernel/events/core.c

+16 −13
Original line number Diff line number Diff line
@@ -752,24 +752,21 @@ perf_cgroup_mark_enabled(struct perf_event *event, 
static enum hrtimer_restart perf_mux_hrtimer_handler(struct hrtimer *hr)

{

	struct perf_cpu_context *cpuctx;

	enum hrtimer_restart ret = HRTIMER_NORESTART;

	int rotations = 0;



	WARN_ON(!irqs_disabled());



	cpuctx = container_of(hr, struct perf_cpu_context, hrtimer);



	rotations = perf_rotate_context(cpuctx);



	/*

	 * arm timer if needed

	 */

	if (rotations) {

	raw_spin_lock(&cpuctx->hrtimer_lock);

	if (rotations)

		hrtimer_forward_now(hr, cpuctx->hrtimer_interval);

		ret = HRTIMER_RESTART;

	}

	else

		cpuctx->hrtimer_active = 0;

	raw_spin_unlock(&cpuctx->hrtimer_lock);



	return ret;

	return rotations ? HRTIMER_RESTART : HRTIMER_NORESTART;

}



static void __perf_mux_hrtimer_init(struct perf_cpu_context *cpuctx, int cpu)
@@ -792,7 +789,8 @@ static void __perf_mux_hrtimer_init(struct perf_cpu_context *cpuctx, int cpu) 


	cpuctx->hrtimer_interval = ns_to_ktime(NSEC_PER_MSEC * interval);



	hrtimer_init(timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_PINNED);

	raw_spin_lock_init(&cpuctx->hrtimer_lock);

	hrtimer_init(timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED);

	timer->function = perf_mux_hrtimer_handler;

}
@@ -800,15 +798,20 @@ static int perf_mux_hrtimer_restart(struct perf_cpu_context *cpuctx) 
{

	struct hrtimer *timer = &cpuctx->hrtimer;

	struct pmu *pmu = cpuctx->ctx.pmu;

	unsigned long flags;



	/* not for SW PMU */

	if (pmu->task_ctx_nr == perf_sw_context)

		return 0;



	if (hrtimer_is_queued(timer))

		return 0;

	raw_spin_lock_irqsave(&cpuctx->hrtimer_lock, flags);

	if (!cpuctx->hrtimer_active) {

		cpuctx->hrtimer_active = 1;

		hrtimer_forward_now(timer, cpuctx->hrtimer_interval);

		hrtimer_start_expires(timer, HRTIMER_MODE_ABS_PINNED);

	}

	raw_spin_unlock_irqrestore(&cpuctx->hrtimer_lock, flags);



	hrtimer_start(timer, cpuctx->hrtimer_interval, HRTIMER_MODE_REL_PINNED);

	return 0;

}

kernel/sched/core.c

+0 −12
Original line number Diff line number Diff line
@@ -90,18 +90,6 @@ 
#define CREATE_TRACE_POINTS

#include <trace/events/sched.h>



void start_bandwidth_timer(struct hrtimer *period_timer, ktime_t period)

{

	/*

	 * Do not forward the expiration time of active timers;

	 * we do not want to loose an overrun.

	 */

	if (!hrtimer_active(period_timer))

		hrtimer_forward_now(period_timer, period);



	hrtimer_start_expires(period_timer, HRTIMER_MODE_ABS_PINNED);

}



DEFINE_MUTEX(sched_domains_mutex);

DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);

kernel/sched/fair.c

+13 −4
Original line number Diff line number Diff line
@@ -3870,8 +3870,9 @@ static void start_cfs_slack_bandwidth(struct cfs_bandwidth *cfs_b) 
	if (runtime_refresh_within(cfs_b, min_left))

		return;



	start_bandwidth_timer(&cfs_b->slack_timer,

				ns_to_ktime(cfs_bandwidth_slack_period));

	hrtimer_start(&cfs_b->slack_timer,

			ns_to_ktime(cfs_bandwidth_slack_period),

			HRTIMER_MODE_REL);

}



/* we know any runtime found here is valid as update_curr() precedes return */
@@ -4012,6 +4013,8 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer) 


		idle = do_sched_cfs_period_timer(cfs_b, overrun);

	}

	if (idle)

		cfs_b->period_active = 0;

	raw_spin_unlock(&cfs_b->lock);



	return idle ? HRTIMER_NORESTART : HRTIMER_RESTART;
@@ -4025,7 +4028,7 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b) 
	cfs_b->period = ns_to_ktime(default_cfs_period());



	INIT_LIST_HEAD(&cfs_b->throttled_cfs_rq);

	hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);

	hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED);

	cfs_b->period_timer.function = sched_cfs_period_timer;

	hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);

	cfs_b->slack_timer.function = sched_cfs_slack_timer;
@@ -4039,7 +4042,13 @@ static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq) 


void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)

{

	start_bandwidth_timer(&cfs_b->period_timer, cfs_b->period);

	lockdep_assert_held(&cfs_b->lock);



	if (!cfs_b->period_active) {

		cfs_b->period_active = 1;

		hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);

		hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);

	}

}



static void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b)

kernel/sched/rt.c

+7 −1
Original line number Diff line number Diff line
@@ -31,6 +31,8 @@ static enum hrtimer_restart sched_rt_period_timer(struct hrtimer *timer) 
		idle = do_sched_rt_period_timer(rt_b, overrun);

		raw_spin_lock(&rt_b->rt_runtime_lock);

	}

	if (idle)

		rt_b->rt_period_active = 0;

	raw_spin_unlock(&rt_b->rt_runtime_lock);



	return idle ? HRTIMER_NORESTART : HRTIMER_RESTART;
@@ -54,7 +56,11 @@ static void start_rt_bandwidth(struct rt_bandwidth *rt_b) 
		return;



	raw_spin_lock(&rt_b->rt_runtime_lock);

	start_bandwidth_timer(&rt_b->rt_period_timer, rt_b->rt_period);

	if (!rt_b->rt_period_active) {

		rt_b->rt_period_active = 1;

		hrtimer_forward_now(&rt_b->rt_period_timer, rt_b->rt_period);

		hrtimer_start_expires(&rt_b->rt_period_timer, HRTIMER_MODE_ABS_PINNED);

	}

	raw_spin_unlock(&rt_b->rt_runtime_lock);

}

CRA Git | Maintained and supported by SUSTech CRA and CCSE