linux-kernel - Re: [PATCH] tracing: Explicitly cast divisor to fix Coccinelle warning

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Date: Wed, 20 Mar 2024 17:30:00 +0100
From: Thorsten Blum <thorsten.blum@...lux.com>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: Explicitly cast divisor to fix Coccinelle
 warning

On 20. Mar 2024, at 16:01, Thorsten Blum <thorsten.blum@...lux.com> wrote:
> 
> Coccinelle also finds this one, but please ignore this patch as I just realized
> this was already fixed in another patch of mine from February.
> 
> Sorry for the inconvenience.
> 
> Link: https://lore.kernel.org/linux-kernel/20240225164507.232942-2-thorsten.blum@toblux.com/

Actually, I will submit a new patch to revert

	delta = div64_u64(delta, bm_cnt);

back to

	do_div(delta, bm_cnt);

but this time include an explicit cast to u32

	do_div(delta, (u32)bm_cnt);

to remove the Coccinelle warning reported by do_div.cocci and to improve
performance.

The do_div() macro does a 64-by-32 division which is faster than the 64-by-64 
division done by div64_u64(). Casting the divisor bm_cnt to u32 is safe since 
we return early from trace_do_benchmark() if bm_cnt > UINT_MAX (something I 
missed in d6cb38e10810).

This approach is already used when calculating the standard deviation:

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

Thanks,
Thorsten