On Aug 30, 2021, at 3:22 AM, Peter Zijlstra [off-list ref] wrote:

On Thu, Aug 26, 2021 at 03:13:04PM -0700, Song Liu wrote:

quoted

+int dummy_perf_snapshot_branch_stack(struct perf_branch_snapshot *br_snapshot);
+
+DECLARE_STATIC_CALL(perf_snapshot_branch_stack, dummy_perf_snapshot_branch_stack);
+
#endif /* _LINUX_PERF_EVENT_H */

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 011cc5069b7ba..c53fe90e630ac 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c

@@ -13437,3 +13437,6 @@ struct cgroup_subsys perf_event_cgrp_subsys = {

	.threaded	= true,
};
#endif /* CONFIG_CGROUP_PERF */
+
+DEFINE_STATIC_CALL_NULL(perf_snapshot_branch_stack,
+			dummy_perf_snapshot_branch_stack);

This isn't right...

The whole dummy_perf_snapshot_branch_stack() thing is a declaration only
and used as a typedef. Also, DEFINE_STATIC_CALL_NULL() and
static_call_cond() rely on a void return value, which it doesn't have.

Did you want:

 DECLARE_STATIC_CALL(perf_snapshot_branch_stack, void (*)(struct perf_branch_snapshot *));

 DEFINE_STATIC_CALL_NULL(perf_snapshot_branch_stack, void (*)(struct perf_branch_snapshot *));

 static_call_cond(perf_snapshot_branch_stack)(...);

*OR*, do you actually need that return value, in which case you're
probably looking for:

 DECLARE_STATIC_CALL(perf_snapshot_branch_stack, int (*)(struct perf_branch_snapshot *));

 DEFINE_STATIC_CALL_RET0(perf_snapshot_branch_stack, int (*)(struct perf_branch_snapshot *));

 ret = static_call(perf_snapshot_branch_stack)(...);

?

Hmmm... something doesn't work here. I have:

/* include/linux/perf_event.h */
DECLARE_STATIC_CALL(perf_snapshot_branch_stack,
                   int (*)(struct perf_branch_snapshot *));


/* kernel/events/core.c */
DEFINE_STATIC_CALL_RET0(perf_snapshot_branch_stack,
                       int (*)(struct perf_branch_snapshot *));

/* kernel/bpf/trampoline.c */
       if (prog->call_get_branch)
               static_call(perf_snapshot_branch_stack)(
                       this_cpu_ptr(&bpf_perf_branch_snapshot));

/* arch/x86/events/intel/core.c */
       if (x86_pmu.disable_all == intel_pmu_disable_all)
               static_call_update(perf_snapshot_branch_stack,
                                  intel_pmu_snapshot_branch_stack);

And the compiler keeps complain with:

arch/x86/events/intel/core.c: In function ‘intel_pmu_init’:
./include/linux/static_call.h:121:41: error: initialization of ‘int (**)(struct perf_branch_snapshot *)’ from incompatible pointer type ‘int (*)(struct perf_branch_snapshot *)’ [-Werror=incompatible-pointer-type ]
  typeof(&STATIC_CALL_TRAMP(name)) __F = (func);   \
                                         ^
arch/x86/events/intel/core.c:6305:4: note: in expansion of macro ‘static_call_update’
    static_call_update(perf_snapshot_branch_stack,
    ^~~~~~~~~~~~~~~~~~


Something like 

typedef int (perf_snapshot_branch_stack_t)(struct perf_branch_snapshot *);
DECLARE_STATIC_CALL(perf_snapshot_branch_stack, perf_snapshot_branch_stack_t);

seems to work fine. 

Thanks,
Song