[PATCH bpf-next] bpf: Improve program stats run-time calculation

[Jose Fernandez]

This patch improves the run-time calculation for program stats by
capturing the duration as soon as possible after the program returns.
Previously, the duration included u64_stats_t operations. While the
instrumentation overhead is part of the total time spent when stats are
enabled, distinguishing between the program's native execution time and
the time spent due to instrumentation is crucial for accurate
performance analysis. By making this change, the patch facilitates more
precise optimization of BPF programs, enabling users to understand their
performance in environments without stats enabled.

I used a virtualized environment to measure the run-time over one minute
for a basic raw_tracepoint/sys_enter program, which just increments a
local counter. Although the virtualization introduced some performance
degradation that could affect the results, I observed approximately a
16% decrease in average run-time reported by stats with this change
(310 -> 260 nsec).

Signed-off-by: Jose Fernandez <josef@xxxxxxxxxxx>
---
 include/linux/filter.h  | 5 ++++-
 kernel/bpf/trampoline.c | 3 ++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 44934b968b57..2a8eb6fe2489 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -654,14 +654,17 @@ static __always_inline u32 __bpf_prog_run(const struct bpf_prog *prog,
 	cant_migrate();
 	if (static_branch_unlikely(&bpf_stats_enabled_key)) {
 		struct bpf_prog_stats *stats;
+		u64 duration;
 		u64 start = sched_clock();
 		unsigned long flags;
 
 		ret = dfunc(ctx, prog->insnsi, prog->bpf_func);
+
+		duration = sched_clock() - start;
 		stats = this_cpu_ptr(prog->stats);
 		flags = u64_stats_update_begin_irqsave(&stats->syncp);
 		u64_stats_inc(&stats->cnt);
-		u64_stats_add(&stats->nsecs, sched_clock() - start);
+		u64_stats_add(&stats->nsecs, duration);
 		u64_stats_update_end_irqrestore(&stats->syncp, flags);
 	} else {
 		ret = dfunc(ctx, prog->insnsi, prog->bpf_func);
diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index cc50607f8d8c..778453137b07 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -886,11 +886,12 @@ static void notrace update_prog_stats(struct bpf_prog *prog,
 	     */
 	    start > NO_START_TIME) {
 		unsigned long flags;
+		u64 duration = sched_clock() - start;
 
 		stats = this_cpu_ptr(prog->stats);
 		flags = u64_stats_update_begin_irqsave(&stats->syncp);
 		u64_stats_inc(&stats->cnt);
-		u64_stats_add(&stats->nsecs, sched_clock() - start);
+		u64_stats_add(&stats->nsecs, duration);
 		u64_stats_update_end_irqrestore(&stats->syncp, flags);
 	}
 }

base-commit: 623bdd58be3727318d374f0052f9dfff1e87b854
-- 
2.34.1