bpf: Improve program stats run-time calculation

[ Upstream commit ce09cbdd988887662546a1175bcfdfc6c8fdd150 ]

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@netflix.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240402034010.25060-1-josef@netflix.com
Stable-dep-of: 7dc211c1159d ("bpf: Fix invalid prog->stats access when update_effective_progs fails")
Signed-off-by: Sasha Levin <sashal@kernel.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/include/linux/filter.h b/include/linux/filter.h
index ad5a3d68b555..f9fb83be935d 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -599,14 +599,16 @@ 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 start = sched_clock();
+		u64 duration, 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);