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Date: Sun, 23 Aug 2015 09:01:01 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Frederic Weisbecker <fweisbec@...il.com>,
Peter Zijlstra <peterz@...radead.org>,
LKML <linux-kernel@...r.kernel.org>,
Vatika Harlalka <vatikaharlalka@...il.com>,
Chris Metcalf <cmetcalf@...hip.com>,
Thomas Gleixner <tglx@...utronix.de>,
Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers
On Sun, Aug 23, 2015 at 07:40:32AM +0200, Ingo Molnar wrote:
>
> * Frederic Weisbecker <fweisbec@...il.com> wrote:
>
> > From: Vatika Harlalka <vatikaharlalka@...il.com>
> >
> > The problem addressed in this patch is about affining unpinned timers.
> > Adaptive or Full Dynticks CPUs are currently disturbed by unnecessary
> > jitter due to firing of such timers on them.
> >
> > This patch will affine timers to online CPUs which are not full dynticks
> > in NOHZ_FULL configured systems. It should not introduce overhead in
> > nohz full off case due to static keys.
> >
> > Reviewed-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> > Signed-off by: Vatika Harlalka <vatikaharlalka@...il.com>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Christoph Lameter <cl@...ux.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Chris Metcalf <cmetcalf@...hip.com>
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > ---
> > include/linux/tick.h | 9 ++++++++-
> > kernel/sched/core.c | 7 +++++--
> > 2 files changed, 13 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/tick.h b/include/linux/tick.h
> > index 3741ba1..51e6493 100644
> > --- a/include/linux/tick.h
> > +++ b/include/linux/tick.h
> > @@ -143,13 +143,20 @@ static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask)
> > if (tick_nohz_full_enabled())
> > cpumask_or(mask, mask, tick_nohz_full_mask);
> > }
> > -
> > +static inline int housekeeping_any_cpu(void)
> > +{
> > + return cpumask_any_and(housekeeping_mask, cpu_online_mask);
> > +}
> > extern void __tick_nohz_full_check(void);
> > extern void tick_nohz_full_kick(void);
> > extern void tick_nohz_full_kick_cpu(int cpu);
> > extern void tick_nohz_full_kick_all(void);
> > extern void __tick_nohz_task_switch(struct task_struct *tsk);
> > #else
> > +static inline int housekeeping_any_cpu(void)
> > +{
> > + return smp_processor_id();
> > +}
> > static inline bool tick_nohz_full_enabled(void) { return false; }
> > static inline bool tick_nohz_full_cpu(int cpu) { return false; }
> > static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask) { }
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 9917c96..4fd42e4 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -623,18 +623,21 @@ int get_nohz_timer_target(void)
> > int i, cpu = smp_processor_id();
> > struct sched_domain *sd;
> >
> > - if (!idle_cpu(cpu))
> > + if (!idle_cpu(cpu) && is_housekeeping_cpu(cpu))
> > return cpu;
> >
> > rcu_read_lock();
> > for_each_domain(cpu, sd) {
> > for_each_cpu(i, sched_domain_span(sd)) {
> > - if (!idle_cpu(i)) {
> > + if (!idle_cpu(i) && is_housekeeping_cpu(cpu)) {
> > cpu = i;
> > goto unlock;
> > }
> > }
> > }
> > +
> > + if (!is_housekeeping_cpu(cpu))
> > + cpu = housekeeping_any_cpu();
> > unlock:
> > rcu_read_unlock();
> > return cpu;
>
> So I almost applied this yesterday, but had the following question: what ensures
> that housekeeping_mask isn't empty? If it's empty then housekeeping_any_cpu()
> returns cpumask_any_and() of an empty cpumask - which returns an out of range
> index AFAICS - which will crash and burn in:
>
> kernel/time/hrtimer.c: return &per_cpu(hrtimer_bases, get_nohz_timer_target());
> kernel/time/timer.c: return per_cpu_ptr(&tvec_bases, get_nohz_timer_target());
>
> housekeeping_mask itself is derived from tick_nohz_full_mask (it's the inverse of
> it in essence), and tick_nohz_full_mask is set via two methods, either via a boot
> parameter:
>
> if (cpulist_parse(str, tick_nohz_full_mask) < 0) {
>
> in tick_nohz_full_setup(). What ensures here that tick_nohz_full_mask is not
> completely full - making housekeeping_mask empty?
>
> The other method is via CONFIG_NO_HZ_FULL_ALL:
>
> cpumask_setall(tick_nohz_full_mask);
>
> here it's fully set - triggering the bug I'm worried about. So what am I missing,
> what prevents CONFIG_NO_HZ_FULL_ALL from crashing?
The boot CPU is excluded from tick_nohz_full_mask in tick_nohz_init(),
which is called from tick_init() which is called from start_kernel()
shortly after rcu_init():
cpu = smp_processor_id();
if (cpumask_test_cpu(cpu, tick_nohz_full_mask)) {
pr_warning("NO_HZ: Clearing %d from nohz_full range for timekeeping\n", cpu);
cpumask_clear_cpu(cpu, tick_nohz_full_mask);
}
This happens after the call to tick_nohz_init_all() that does the
cpumask_setall() that you called out above.
Or is a recent patch that I missed changing this?
Thanx, Paul
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