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

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Message-ID: <CADxym3Za-zShEUyoVE7OoODKYXc1nghD63q2xv_wtHAyT2-Z-Q@mail.gmail.com>
Date: Mon, 28 Jul 2025 17:20:16 +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 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 :/

>
> 4.
> Maybe we should extend clang/gcc to support attr(preserve_access_index)
> on x86 and other architectures ;)
> We rely heavily on it in bpf backend.
> Then one can simply write:
>
> struct rq___my {
>   unsigned int nr_pinned;
> } __attribute__((preserve_access_index));
>
> struct rq___my *rq;
>
> rq = this_rq();
> rq->nr_pinned++;
>
> and the compiler will do its magic of offset adjustment.
> That's how BPF CORE works.