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

[Leonardo Bras]

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=?, 
so user could disable qpw if wanted.

Would that work?

Thanks!
Leo