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Subject: Re: [PATCH] tracing: Explicitly cast divisor to fix Coccinelle
 warning
From: Thorsten Blum <thorsten.blum@toblux.com>
In-Reply-To: <EAA3D15D-F96D-48AC-B9C0-B2290E1B38AA@toblux.com>
Date: Wed, 20 Mar 2024 17:30:00 +0100
Cc: Steven Rostedt <rostedt@goodmis.org>,
 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
 linux-kernel@vger.kernel.org,
 linux-trace-kernel@vger.kernel.org
Content-Transfer-Encoding: quoted-printable
Message-Id: <4E5A47C3-E9DD-4FD2-BDC4-BED67EF791F2@toblux.com>
References: <20240318105243.117911-2-thorsten.blum@toblux.com>
 <20240320192715.22eeeba84c0fd3a8c2353c79@kernel.org>
 <EAA3D15D-F96D-48AC-B9C0-B2290E1B38AA@toblux.com>
To: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>

On 20. Mar 2024, at 16:01, Thorsten Blum <thorsten.blum@toblux.com> =
wrote:
>=20
> 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.
>=20
> Sorry for the inconvenience.
>=20
> Link: =
https://lore.kernel.org/linux-kernel/20240225164507.232942-2-thorsten.blum=
@toblux.com/

Actually, I will submit a new patch to revert

	delta =3D 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=20
division done by div64_u64(). Casting the divisor bm_cnt to u32 is safe =
since=20
we return early from trace_do_benchmark() if bm_cnt > UINT_MAX =
(something I=20
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=