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

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Message-ID: <CAADnVQ+5sEDKHdsJY5ZsfGDO_1SEhhQWHrt2SMBG5SYyQ+jt7w@mail.gmail.com>
Date: Wed, 16 Jul 2025 15:35:16 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Menglong Dong <menglong.dong@...ux.dev>, Menglong Dong <menglong8.dong@...il.com>, 
	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 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.


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.