On Mon, Jul 28, 2025 at 2:20 AM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
>
> On Thu, Jul 17, 2025 at 6:35 AM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > On Wed, Jul 16, 2025 at 11:24 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > On Wed, Jul 16, 2025 at 09:56:11AM -0700, Alexei Starovoitov wrote:
> > >
> > > > Maybe Peter has better ideas ?
> > >
> > > Is it possible to express runqueues::nr_pinned as an alias?
> > >
> > > extern unsigned int __attribute__((alias("runqueues.nr_pinned"))) this_nr_pinned;
> > >
> > > And use:
> > >
> > > __this_cpu_inc(&this_nr_pinned);
> > >
> > >
> > > This syntax doesn't actually seem to work; but can we construct
> > > something like that?
> >
> > Yeah. Iant is right. It's a string and not a pointer dereference.
> > It never worked.
> >
> > Few options:
> >
> > 1.
> > struct rq {
> > +#ifdef CONFIG_SMP
> > + unsigned int nr_pinned;
> > +#endif
> > /* runqueue lock: */
> > raw_spinlock_t __lock;
> >
> > @@ -1271,9 +1274,6 @@ struct rq {
> > struct cpuidle_state *idle_state;
> > #endif
> >
> > -#ifdef CONFIG_SMP
> > - unsigned int nr_pinned;
> > -#endif
> >
> > but ugly...
> >
> > 2.
> > static unsigned int nr_pinned_offset __ro_after_init __used;
> > RUNTIME_CONST(nr_pinned_offset, nr_pinned_offset)
> >
> > overkill for what's needed
> >
> > 3.
> > OFFSET(RQ_nr_pinned, rq, nr_pinned);
> > then
> > #include <generated/asm-offsets.h>
> >
> > imo the best.
>
> I had a try. The struct rq is not visible to asm-offsets.c, so we
> can't define it in arch/xx/kernel/asm-offsets.c. Do you mean
> to define a similar rq-offsets.c in kernel/sched/ ? It will be more
> complex than the way 2, and I think the second way 2 is
> easier :/
2 maybe easier, but it's an overkill.
I still think asm-offset is cleaner.
arch/xx shouldn't be used, of course, since this nr_pinned should
be generic for all archs.
We can do something similar to drivers/memory/emif-asm-offsets.c
and do that within kernel/sched/.
rq-offsets.c as you said.
It will generate rq-offsets.h in a build dir that can be #include-d.
I thought about another alternative (as a derivative of 1):
split nr_pinned from 'struct rq' into its own per-cpu variable,
but I don't think that will work, since rq_has_pinned_tasks()
doesn't always operate on this_rq().
So the acceptable choices are realistically 1 and 3 and
rq-offsets.c seems cleaner.
Pls give it another try.
