linux-kernel - [for-next][PATCH 3/7] tracing: Improve benchmark test performance by using do

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Date: Tue, 14 May 2024 12:14:41 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: linux-kernel@...r.kernel.org
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Thorsten Blum <thorsten.blum@...lux.com>
Subject: [for-next][PATCH 3/7] tracing: Improve benchmark test performance by using do_div()

From: Thorsten Blum <thorsten.blum@...lux.com>

Partially revert commit d6cb38e10810 ("tracing: Use div64_u64() instead
of do_div()") and use do_div() again to utilize its faster 64-by-32
division compared to the 64-by-64 division done by div64_u64().

Explicitly cast the divisor bm_cnt to u32 to prevent a Coccinelle
warning reported by do_div.cocci. The warning was removed with commit
d6cb38e10810 ("tracing: Use div64_u64() instead of do_div()").

Using the faster 64-by-32 division and casting bm_cnt to u32 is safe
because we return early from trace_do_benchmark() if bm_cnt > UINT_MAX.
This approach is already used twice in trace_do_benchmark() when
calculating the standard deviation:

	do_div(stddev, (u32)bm_cnt);
	do_div(stddev, (u32)bm_cnt - 1);

Link: https://lore.kernel.org/linux-trace-kernel/20240329160229.4874-2-thorsten.blum@toblux.com

Signed-off-by: Thorsten Blum <thorsten.blum@...lux.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 kernel/trace/trace_benchmark.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace_benchmark.c b/kernel/trace/trace_benchmark.c
index 811b08439406..e19c32f2a938 100644
--- a/kernel/trace/trace_benchmark.c
+++ b/kernel/trace/trace_benchmark.c
@@ -104,7 +104,7 @@ static void trace_do_benchmark(void)
 		stddev = 0;

 	delta = bm_total;
-	delta = div64_u64(delta, bm_cnt);
+	do_div(delta, (u32)bm_cnt);
 	avg = delta;

 	if (stddev > 0) {
-- 
2.43.0