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

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

On Tue, Mar 03, 2026 at 01:02:13PM -0300, Marcelo Tosatti wrote:
> On Tue, Mar 03, 2026 at 01:03:36PM +0100, Vlastimil Babka (SUSE) wrote:
> > On 3/2/26 16:49, Marcelo Tosatti wrote:
> > > +#define local_qpw_lock(lock)								\
> > > +	do {										\
> > > +		if (static_branch_maybe(CONFIG_QPW_DEFAULT, &qpw_sl)) {			\
> > > +			migrate_disable();						\
> > 
> > Have you considered using migrate_disable() on PREEMPT_RT and
> > preempt_disable() on !PREEMPT_RT since it's cheaper? It's what the pcp
> > locking in mm/page_alloc.c does, for that reason. It should reduce the
> > overhead with qpw=1 on !PREEMPT_RT.
> 
> migrate_disable:
> Patched kernel, CONFIG_QPW=y, qpw=1:    192 cycles
> 
> preempt_disable:
> [   65.497223] kmalloc_bench: Avg cycles per kmalloc: 184 cycles
> 
> I tried it before, but it was crashing for some reason which i didnt
> look into (perhaps PREEMPT_RT was enabled).
> 
> Will change this for the next iteration, thanks.
> 

Hi all,

That made me remember that rt spinlock already uses migrate_disable and 
non-rt spinlocks already have preempt_disable()

Maybe it's actually worth adding a local_spin_lock() in spinlock{,_rt}.c 
whichy would get the per-cpu variable inside the preempt/migrate_disable 
area, and making use of it in qpw code. That way we avoid nesting 
migtrate_disable or preempt_disable, and further reducing impact.

The alternative is to not have migrate/preempt disable here and actually 
trust the ones inside the locking primitives. Is there a chance of 
contention, but I don't remember being able to detect it.

Thanks!
Leo