Re: [PATCH v2 2/5] Introducing qpw_lock() and per-cpu queue & flush work

[Leonardo Bras <leobras.c@gmail.com>]

On Mon, Mar 16, 2026 at 11:55:46AM +0100, Vlastimil Babka (SUSE) wrote:
> On 3/15/26 18:37, Leonardo Bras wrote:
> > On Wed, Mar 11, 2026 at 08:58:05AM +0100, Vlastimil Babka (SUSE) wrote:
> >> On 3/2/26 16:49, Marcelo Tosatti wrote:
> >> > Index: linux/Documentation/admin-guide/kernel-parameters.txt
> >> > ===================================================================
> >> > --- linux.orig/Documentation/admin-guide/kernel-parameters.txt
> >> > +++ linux/Documentation/admin-guide/kernel-parameters.txt
> >> > @@ -2840,6 +2840,16 @@ Kernel parameters
> >> >  
> >> >  			The format of <cpu-list> is described above.
> >> >  
> >> > +	qpw=		[KNL,SMP] Select a behavior on per-CPU resource sharing
> >> > +			and remote interference mechanism on a kernel built with
> >> > +			CONFIG_QPW.
> >> > +			Format: { "0" | "1" }
> >> > +			0 - local_lock() + queue_work_on(remote_cpu)
> >> > +			1 - spin_lock() for both local and remote operations
> >> > +
> >> > +			Selecting 1 may be interesting for systems that want
> >> > +			to avoid interruption & context switches from IPIs.
> >> Requiring a new boot option is always a nuissance. The cpu isolation is
> >> AFAIK difficult enough to setup already. Could the default be that qpw will
> >> auto-enable if there are isolated cpus configured? The option could still be
> >> useful for overriding that automatic decision to both 0 and 1 for testing
> >> etc, but not requried for the expected usecase?
> > 
> > 
> > I think it's okay, as something like this?
> > (should work for nohz_full and isolcpus)
> > 
> > ######
> > diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> > index 81bc8b329ef17..6c9052c28e3e4 100644
> > --- a/kernel/sched/isolation.c
> > +++ b/kernel/sched/isolation.c
> > @@ -170,20 +170,23 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
> >                 for_each_set_bit(type, &iter_flags, HK_TYPE_MAX)
> >                         housekeeping_setup_type(type, housekeeping_staging);
> >         }
> >  
> >         if ((flags & HK_FLAG_KERNEL_NOISE) && !(housekeeping.flags & HK_FLAG_KERNEL_NOISE))
> >                 tick_nohz_full_setup(non_housekeeping_mask);
> >  
> >         housekeeping.flags |= flags;
> >         err = 1;
> >  
> > +       if (IS_ENABLED(CONFIG_QPW_DEFAULT))
> > +               qpw_setup("1");
> > +
> >  free_housekeeping_staging:
> >         free_bootmem_cpumask_var(housekeeping_staging);
> >  free_non_housekeeping_mask:
> >         free_bootmem_cpumask_var(non_housekeeping_mask);
> >  
> >         return err;
> >  }
> > ######
> > 
> > We would only have to be sure that this runs before cmdline parses qpw=?, 
> 
> I'm not sure it's possible to achieve this ordering with __setup calls,
> unless one of them is early, and then it might be too early to do the
> necessary action.
> 
> > so user could disable qpw if wanted.
> > 
> > Would that work?
> 
> The pattern I'm familiar with is collecting all related params via
> early_param() setting some variables, and then an init call (not tied to any
> of the param) looks at those variables and does whatever is necessary.
> 
> > Thanks!
> > Leo
> > 
> > 
> > 
> 

Makes sense, will take a look on that approach.

Thanks!
Leo