linux-kernel - Re: [PATCH v9 6/8] sched/isolation: Force housekeeping if isolcpus and nohz

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Message-ID: <1d12942942150462f77ea87fec8f294f46c87b4f.camel@redhat.com>
Date: Fri, 01 Aug 2025 16:46:26 +0200
From: Gabriele Monaco <gmonaco@...hat.com>
To: Waiman Long <llong@...hat.com>, Frederic Weisbecker <frederic@...nel.org>
Cc: linux-kernel@...r.kernel.org, Anna-Maria Behnsen
 <anna-maria@...utronix.de>,  Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v9 6/8] sched/isolation: Force housekeeping if isolcpus
 and nohz_full don't leave any



On Thu, 2025-07-31 at 11:09 -0400, Waiman Long wrote:
> 
> On 7/30/25 9:11 AM, Gabriele Monaco wrote:
> > Currently the user can set up isolcpus and nohz_full in such a way
> > that
> > leaves no housekeeping CPU (i.e. no CPU that is neither domain
> > isolated
> > nor nohz full). This can be a problem for other subsystems (e.g.
> > the
> > timer wheel imgration).
> > 
> > Prevent this configuration by invalidating the last setting in case
> > the
> > union of isolcpus and nohz_full covers all CPUs.
> > 
> > Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
> > Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>
> > ---
> >   kernel/sched/isolation.c | 12 ++++++++++++
> >   1 file changed, 12 insertions(+)
> > 
> > diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> > index 93b038d48900..0019d941de68 100644
> > --- a/kernel/sched/isolation.c
> > +++ b/kernel/sched/isolation.c
> > @@ -165,6 +165,18 @@ static int __init housekeeping_setup(char
> > *str, unsigned long flags)
> >   			}
> >   		}
> >   
> > +		/* Check in combination with the previously set
> > cpumask */
> > +		type = find_first_bit(&housekeeping.flags,
> > HK_TYPE_MAX);
> > +		first_cpu =
> > cpumask_first_and_and(cpu_present_mask,
> > +						 
> > housekeeping_staging,
> > +						 
> > housekeeping.cpumasks[type]);
> > +		if (first_cpu >= nr_cpu_ids || first_cpu >=
> > setup_max_cpus) {
> > +			pr_warn("Housekeeping: must include one
> > present CPU neither "
> > +				"in nohz_full= nor in isolcpus=,
> > ignoring setting %s\n",
> > +				str);
> > +			goto free_housekeeping_staging;
> > +		}
> > +
> >   		iter_flags = flags & ~housekeeping.flags;
> >   
> >   		for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
> 
> I do have a question about this check. Currently isolcpus=domain is
> bit 0, managed_irq is bit 1 and nohz_full is bit 2. If manage_irq
> come first followed by nohz_full and then isolcpus=domain. By the
> time, isolcpus=domain is being set, you are comparing its cpumask
> with that of manage_irq, not nohz_full.
> 
> Perhaps you can reuse the non_housekeeping_mask for doing the check,
> e.g.
> 
>          cpumask_and(non_housekeeping_mask, cpu_present_mask, 
> housekeeping_staging);
>          iter_flags = housekeeping.flags & ~flags;
>          for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
>                  cpumask_and(non_housekeeping_mask, 
> non_housekeeping_mask, housekeeping.cpumasks[type]);
>          if (cpumask_empty(non_housekeeping_mask)) {
>                  pr_warn(...

Mmh right didn't think passing different masks in isocpus was possible.

You mean something like this right?

 isolcpus=managed_irq,0-4 nohz_full=8-15 isolcpus=domain,0-7

Which doesn't block the nohz_full because the first mask (managed_irq)
leaves spaces.

Right now we block assignments like

 isolcpus=managed_irq,0-7 nohz_full=8-15

and 

 isolcpus=managed_irq,0-7 -a isolcpus=domain,8-15

although this series doesn't really have problems with it.
Shouldn't we just ignore these cases and just count domain + nohz_full?

The solution you propose is to check all housekeeping, so it would also
prevent the (safe?) assignments above, right?

We could just check against the previously set domain/nohz_full and
leave other flags alone, couldn't we?

Thanks,
Gabriele