Re: [PATCH] sched/isolation: fix boot crash when the boot CPU is nohz_full

[Phil Auld]

HI Oleg, Ingo,

On Mon, Apr 22, 2024 at 08:50:21PM +0200 Oleg Nesterov wrote:
> Phil, Frederic,
> 
> Thanks for your review! Who do you think can take these patches?
> At least the 1st one.
> 
> To remind, there are more problems with boot CPU in nohz mask, but
> can we at least fix the kernel crash?
>

I think that would be good.  I don't know if Peter is at full strength.

Ingo could you take look at this, please?


Cheers,
Phil


> Oleg.
> 
> On 04/18, Phil Auld wrote:
> >
> > On Thu, Apr 11, 2024 at 04:39:05PM +0200 Oleg Nesterov wrote:
> > > Documentation/timers/no_hz.rst states that the "nohz_full=" mask must not
> > > include the boot CPU, this is no longer true after the commit 08ae95f4fd3b
> > > ("nohz_full: Allow the boot CPU to be nohz_full").
> > >
> > > However after another commit aae17ebb53cd ("workqueue: Avoid using isolated
> > > cpus' timers on queue_delayed_work") the kernel will crash at boot time in
> > > this case; housekeeping_any_cpu() returns an invalid cpu nr until smp_init()
> > > paths bring the 1st housekeeping CPU up.
> > >
> > > Change housekeeping_any_cpu() to check the result of cpumask_any_and() and
> > > return smp_processor_id() in this case. Yes, this is just the simple and
> > > backportable workaround which fixes the symptom, but smp_processor_id() at
> > > boot time should be safe at least for type == HK_TYPE_TIMER, this more or
> > > less matches the tick_do_timer_boot_cpu logic.
> > >
> > > We should not worry about cpu_down(); tick_nohz_cpu_down() will not allow
> > > to offline tick_do_timer_cpu (the 1st online housekeeping CPU).
> > >
> > > Fixes: aae17ebb53cd ("workqueue: Avoid using isolated cpus' timers on queue_delayed_work")
> > > Reported-by: Chris von Recklinghausen <crecklin@xxxxxxxxxx>
> > > Closes: https://lore.kernel.org/all/20240402105847.GA24832@xxxxxxxxxx/
> > > Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> >
> > Checking the returned value instead of assuming seems safer in any case.
> >
> > Reviewed-by: Phil Auld <pauld@xxxxxxxxxx>
> >
> > > ---
> > >  Documentation/timers/no_hz.rst |  7 ++-----
> > >  kernel/sched/isolation.c       | 11 ++++++++++-
> > >  2 files changed, 12 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/Documentation/timers/no_hz.rst b/Documentation/timers/no_hz.rst
> > > index f8786be15183..7fe8ef9718d8 100644
> > > --- a/Documentation/timers/no_hz.rst
> > > +++ b/Documentation/timers/no_hz.rst
> > > @@ -129,11 +129,8 @@ adaptive-tick CPUs:  At least one non-adaptive-tick CPU must remain
> > >  online to handle timekeeping tasks in order to ensure that system
> > >  calls like gettimeofday() returns accurate values on adaptive-tick CPUs.
> > >  (This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no running
> > > -user processes to observe slight drifts in clock rate.)  Therefore, the
> > > -boot CPU is prohibited from entering adaptive-ticks mode.  Specifying a
> > > -"nohz_full=" mask that includes the boot CPU will result in a boot-time
> > > -error message, and the boot CPU will be removed from the mask.  Note that
> > > -this means that your system must have at least two CPUs in order for
> > > +user processes to observe slight drifts in clock rate.) Note that this
> > > +means that your system must have at least two CPUs in order for
> > >  CONFIG_NO_HZ_FULL=y to do anything for you.
> > >
> > >  Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded.
> > > diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> > > index 373d42c707bc..2a262d3ecb3d 100644
> > > --- a/kernel/sched/isolation.c
> > > +++ b/kernel/sched/isolation.c
> > > @@ -46,7 +46,16 @@ int housekeeping_any_cpu(enum hk_type type)
> > >  			if (cpu < nr_cpu_ids)
> > >  				return cpu;
> > >
> > > -			return cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
> > > +			cpu = cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
> > > +			if (likely(cpu < nr_cpu_ids))
> > > +				return cpu;
> > > +			/*
> > > +			 * Unless we have another problem this can only happen
> > > +			 * at boot time before start_secondary() brings the 1st
> > > +			 * housekeeping CPU up.
> > > +			 */
> > > +			WARN_ON_ONCE(system_state == SYSTEM_RUNNING ||
> > > +				     type != HK_TYPE_TIMER);
> > >  		}
> > >  	}
> > >  	return smp_processor_id();
> > > --
> > > 2.25.1.362.g51ebf55
> > >
> > >
> > >
> >
> > --
> >
> 

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