linux-kernel - [PATCH] uprobes: get rid of bogus trace

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Message-ID: <20240805202803.1813090-1-andrii@kernel.org>
Date: Mon,  5 Aug 2024 13:28:03 -0700
From: Andrii Nakryiko <andrii@...nel.org>
To: linux-trace-kernel@...r.kernel.org,
	rostedt@...dmis.org,
	mhiramat@...nel.org
Cc: peterz@...radead.org,
	oleg@...hat.com,
	bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	jolsa@...nel.org,
	Andrii Nakryiko <andrii@...nel.org>
Subject: [PATCH] uprobes: get rid of bogus trace_uprobe hit counter

trace_uprobe->nhit counter is not incremented atomically, so its value
is bogus in practice. On the other hand, it's actually a pretty big
uprobe scalability problem due to heavy cache line bouncing between CPUs
triggering the same uprobe.

Drop it and emit obviously unrealistic value in its stead in
uporbe_profiler seq file.

The alternative would be allocating per-CPU counter, but I'm not sure
it's justified.

Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
---
 kernel/trace/trace_uprobe.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
index 52e76a73fa7c..5d38207db479 100644
--- a/kernel/trace/trace_uprobe.c
+++ b/kernel/trace/trace_uprobe.c
@@ -62,7 +62,6 @@ struct trace_uprobe {
 	struct uprobe			*uprobe;
 	unsigned long			offset;
 	unsigned long			ref_ctr_offset;
-	unsigned long			nhit;
 	struct trace_probe		tp;
 };
 
@@ -821,7 +820,7 @@ static int probes_profile_seq_show(struct seq_file *m, void *v)
 
 	tu = to_trace_uprobe(ev);
 	seq_printf(m, "  %s %-44s %15lu\n", tu->filename,
-			trace_probe_name(&tu->tp), tu->nhit);
+		   trace_probe_name(&tu->tp), ULONG_MAX);
 	return 0;
 }
 
@@ -1507,7 +1506,6 @@ static int uprobe_dispatcher(struct uprobe_consumer *con, struct pt_regs *regs)
 	int ret = 0;
 
 	tu = container_of(con, struct trace_uprobe, consumer);
-	tu->nhit++;
 
 	udd.tu = tu;
 	udd.bp_addr = instruction_pointer(regs);
-- 
2.43.5