netdev - Re: Inlining migrate_disable/enable. Was: [PATCH bpf-next v2 02/18] x86,bpf: add bpf_global

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Message-ID: <CADxym3arEsBB-b0Hr52pcwH7H+Lgg6-NKYczPn6W49WRND-UJg@mail.gmail.com>
Date: Fri, 1 Aug 2025 09:42:57 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Menglong Dong <menglong.dong@...ux.dev>, 
	Steven Rostedt <rostedt@...dmis.org>, Jiri Olsa <jolsa@...nel.org>, bpf <bpf@...r.kernel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, 
	LKML <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>
Subject: Re: Inlining migrate_disable/enable. Was: [PATCH bpf-next v2 02/18]
 x86,bpf: add bpf_global_caller for global trampoline

On Fri, Aug 1, 2025 at 12:15 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Mon, Jul 28, 2025 at 2:20 AM Menglong Dong <menglong8.dong@...il.com> wrote:
> >
> > On Thu, Jul 17, 2025 at 6:35 AM Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> > >
> > > On Wed, Jul 16, 2025 at 11:24 AM Peter Zijlstra <peterz@...radead.org> 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

Great, I'll have a try on this way!

> 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.