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Date: Tue, 8 Jan 2019 10:06:25 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Mike Galbraith <efault@....de>
Cc: Tom Putzeys <tom.putzeys@...atlascopco.com>,
"mingo@...hat.com" <mingo@...hat.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: CFS scheduler: spin_lock usage causes dead lock when
smp_apic_timer_interrupt occurs
On Tue, Jan 08, 2019 at 06:30:59AM +0100, Mike Galbraith wrote:
> On Mon, 2019-01-07 at 13:52 +0100, Peter Zijlstra wrote:
> > On Mon, Jan 07, 2019 at 01:28:34PM +0100, Mike Galbraith wrote:
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 960ad0ce77d7..420624c49f38 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -5007,9 +5007,9 @@ 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_ABS_PINNED);
> > > + hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED_HARD);
> > > cfs_b->period_timer.function = sched_cfs_period_timer;
> > > - hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> > > + hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
> > > cfs_b->slack_timer.function = sched_cfs_slack_timer;
> > > }
> >
> > Right, that should sort it. But I'm not sure this is the best solution
> > though. That cfs-runtime crud can (IIRC) iterate lists etc.. so running
> > it from the softirq isn't a bad idea. We just need to fix that locking
> > up a bit.
> >
> > Something a wee bit like so perhaps..
>
> I plugged that into 4.19-rt along with revert of hard irq context, and
> it (as expected) does the trick.
Much appreciated; I queued it as the below.
---
Subject: sched/fair: Robustify CFS-bandwidth timer locking
From: Peter Zijlstra <peterz@...radead.org>
Date: Mon, 7 Jan 2019 13:52:31 +0100
Traditionally hrtimer callbacks were run with IRQs disabled, but with
the introduction of HRTIMER_MODE_SOFT it is possible they run from
SoftIRQ context, which does _NOT_ have IRQs disabled.
Allow for the CFS bandwidth timers (period_timer and slack_timer) to
be ran from SoftIRQ context; this entails removing the assumption that
IRQs are already disabled from the locking.
While mainline doesn't strictly need this, -RT forces all timers not
explicitly marked with MODE_HARD into MODE_SOFT and trips over this.
And marking these timers as MODE_HARD doesn't make sense as they're
not required for RT operation and can potentially be quite expensive.
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Reported-by: Tom Putzeys <tom.putzeys@...atlascopco.com>
Tested-by: Mike Galbraith <efault@....de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20190107125231.GE14122@hirez.programming.kicks-ass.net
---
kernel/sched/fair.c | 30 ++++++++++++++++--------------
1 file changed, 16 insertions(+), 14 deletions(-)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4566,7 +4566,7 @@ static u64 distribute_cfs_runtime(struct
struct rq *rq = rq_of(cfs_rq);
struct rq_flags rf;
- rq_lock(rq, &rf);
+ rq_lock_irqsave(rq, &rf);
if (!cfs_rq_throttled(cfs_rq))
goto next;
@@ -4583,7 +4583,7 @@ static u64 distribute_cfs_runtime(struct
unthrottle_cfs_rq(cfs_rq);
next:
- rq_unlock(rq, &rf);
+ rq_unlock_irqrestore(rq, &rf);
if (!remaining)
break;
@@ -4599,7 +4599,7 @@ static u64 distribute_cfs_runtime(struct
* period the timer is deactivated until scheduling resumes; cfs_b->idle is
* used to track this state.
*/
-static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
+static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, unsigned long flags)
{
u64 runtime, runtime_expires;
int throttled;
@@ -4641,11 +4641,11 @@ static int do_sched_cfs_period_timer(str
while (throttled && cfs_b->runtime > 0 && !cfs_b->distribute_running) {
runtime = cfs_b->runtime;
cfs_b->distribute_running = 1;
- raw_spin_unlock(&cfs_b->lock);
+ raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
/* we can't nest cfs_b->lock while distributing bandwidth */
runtime = distribute_cfs_runtime(cfs_b, runtime,
runtime_expires);
- raw_spin_lock(&cfs_b->lock);
+ raw_spin_lock_irqsave(&cfs_b->lock, flags);
cfs_b->distribute_running = 0;
throttled = !list_empty(&cfs_b->throttled_cfs_rq);
@@ -4754,17 +4754,18 @@ static __always_inline void return_cfs_r
static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
{
u64 runtime = 0, slice = sched_cfs_bandwidth_slice();
+ unsigned long flags;
u64 expires;
/* confirm we're still not at a refresh boundary */
- raw_spin_lock(&cfs_b->lock);
+ raw_spin_lock_irqsave(&cfs_b->lock, flags);
if (cfs_b->distribute_running) {
- raw_spin_unlock(&cfs_b->lock);
+ raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
return;
}
if (runtime_refresh_within(cfs_b, min_bandwidth_expiration)) {
- raw_spin_unlock(&cfs_b->lock);
+ raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
return;
}
@@ -4775,18 +4776,18 @@ static void do_sched_cfs_slack_timer(str
if (runtime)
cfs_b->distribute_running = 1;
- raw_spin_unlock(&cfs_b->lock);
+ raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
if (!runtime)
return;
runtime = distribute_cfs_runtime(cfs_b, runtime, expires);
- raw_spin_lock(&cfs_b->lock);
+ raw_spin_lock_irqsave(&cfs_b->lock, flags);
if (expires == cfs_b->runtime_expires)
lsub_positive(&cfs_b->runtime, runtime);
cfs_b->distribute_running = 0;
- raw_spin_unlock(&cfs_b->lock);
+ raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
}
/*
@@ -4864,20 +4865,21 @@ static enum hrtimer_restart sched_cfs_pe
{
struct cfs_bandwidth *cfs_b =
container_of(timer, struct cfs_bandwidth, period_timer);
+ unsigned long flags;
int overrun;
int idle = 0;
- raw_spin_lock(&cfs_b->lock);
+ raw_spin_lock_irqsave(&cfs_b->lock, flags);
for (;;) {
overrun = hrtimer_forward_now(timer, cfs_b->period);
if (!overrun)
break;
- idle = do_sched_cfs_period_timer(cfs_b, overrun);
+ idle = do_sched_cfs_period_timer(cfs_b, overrun, flags);
}
if (idle)
cfs_b->period_active = 0;
- raw_spin_unlock(&cfs_b->lock);
+ raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
return idle ? HRTIMER_NORESTART : HRTIMER_RESTART;
}
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