Re: [PATCH bpf-next] bpf: make update_prog_stats always_inline

[Alexei Starovoitov]

On Wed, Jun 18, 2025 at 1:58 AM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
>
> The function update_prog_stats() will be called in the bpf trampoline.
> Make it always_inline to reduce the overhead.

What kind of difference did you measure ?

> Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
> ---
>  kernel/bpf/trampoline.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index c4b1a98ff726..134bcfd00b15 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -911,8 +911,8 @@ static u64 notrace __bpf_prog_enter_recur(struct bpf_prog *prog, struct bpf_tram
>         return bpf_prog_start_time();
>  }
>
> -static void notrace update_prog_stats(struct bpf_prog *prog,
> -                                     u64 start)
> +static __always_inline void notrace update_prog_stats(struct bpf_prog *prog,
> +                                                     u64 start)
>  {

How about the following instead:
diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index c4b1a98ff726..728bb2845f41 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -911,28 +911,23 @@ static u64 notrace __bpf_prog_enter_recur(struct
bpf_prog *prog, struct bpf_tram
     return bpf_prog_start_time();
 }

-static void notrace update_prog_stats(struct bpf_prog *prog,
-                      u64 start)
+static noinline void notrace __update_prog_stats(struct bpf_prog *prog,
+                         u64 start)
 {
     struct bpf_prog_stats *stats;
-
-    if (static_branch_unlikely(&bpf_stats_enabled_key) &&
-        /* static_key could be enabled in __bpf_prog_enter*
-         * and disabled in __bpf_prog_exit*.
-         * And vice versa.
-         * Hence check that 'start' is valid.
-         */
-        start > NO_START_TIME) {
-        u64 duration = sched_clock() - start;
-        unsigned long flags;
-
-        stats = this_cpu_ptr(prog->stats);
-        flags = u64_stats_update_begin_irqsave(&stats->syncp);
-        u64_stats_inc(&stats->cnt);
-        u64_stats_add(&stats->nsecs, duration);
-        u64_stats_update_end_irqrestore(&stats->syncp, flags);
-    }
+    u64 duration = sched_clock() - start;
+    unsigned long flags;
+
+    stats = this_cpu_ptr(prog->stats);
+    flags = u64_stats_update_begin_irqsave(&stats->syncp);
+    u64_stats_inc(&stats->cnt);
+    u64_stats_add(&stats->nsecs, duration);
+    u64_stats_update_end_irqrestore(&stats->syncp, flags);
 }
+#define update_prog_stats(prog, start) \
+    if (static_branch_unlikely(&bpf_stats_enabled_key) && \
+        start > NO_START_TIME) \
+        __update_prog_stats(prog, start)

 static void notrace __bpf_prog_exit_recur(struct bpf_prog *prog, u64 start,
                       struct bpf_tramp_run_ctx *run_ctx)


Maybe
if (start > NO_START_TIME)
should stay within __update_prog_stats().

pls run a few experiments.