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.
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.
