Date: Tue, 27 Dec 2016 22:56:26 +0100
From: Pavel Machek <pavel@ucw.cz>
To: Nicholas Mc Guire <hofrat@osadl.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, Jonathan Corbet <corbet@lwn.net>,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH] doc: add note on usleep_range range
Message-ID: <20161227215626.GA5336@amd>
References: <1481601523-14004-1-git-send-email-hofrat@osadl.org>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha1;
        protocol="application/pgp-signature"; boundary="nFreZHaLTZJo0R7j"
Content-Disposition: inline
In-Reply-To: <1481601523-14004-1-git-send-email-hofrat@osadl.org>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2548
Lines: 73


--nFreZHaLTZJo0R7j
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Tue 2016-12-13 04:58:43, Nicholas Mc Guire wrote:
> useleep_range() with a delta of 0 makes no sense and only prevents the
> timer subsystem from optimizing interrupts. As any user of usleep_range()
> is in non-atomic context the timer jitter is in the range of 10s of=20
> microseconds anyway.
>=20
> This adds a note making it clear that a range of 0 is a bad idea.
>=20
> Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
> ---
>=20
> as of 4.9.0 there are about 20 cases of usleep_ranges() that have=20
> min=3D=3Dmax and none of them really look like they are necessary, so=20
> it does seem like a relatively common misunderstanding worth
> noting in the documentation.
>=20
> Patch is against 4.9.0 (localversion-next is 20161212)
>=20
>  Documentation/timers/timers-howto.txt | 7 +++++++
>  1 file changed, 7 insertions(+)
>=20
> diff --git a/Documentation/timers/timers-howto.txt b/Documentation/timers=
/timers-howto.txt
> index 038f8c7..b5cdf82 100644
> --- a/Documentation/timers/timers-howto.txt
> +++ b/Documentation/timers/timers-howto.txt
> @@ -93,6 +93,13 @@ NON-ATOMIC CONTEXT:
>  			tolerances here are very situation specific, thus it
>  			is left to the caller to determine a reasonable range.
> =20
> +			A range of 0, that is usleep_range(100,100) or the=20
> +			like, do not make sense as this code is in a=20
> +			non-atomic section and a system can not be expected=20
> +			to have jitter 0. For any non-RT code any delta

Would it be possible to fix english here?

"to have zero jitter" at least. I believe it is "does not".

I don't see how atomic vs. non-atomic context makes difference. There
are sources of jitter that affect atomic context...

> +			less than 50 microseconds probably is only preventing
> +			timer subsystem optimization but providing no benefit.

And I don't trust you here. _If_ it prevents timer optimalization,
_then_ it provides benefit, at least in the average case.

									Pavel
--=20
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo=
g.html

--nFreZHaLTZJo0R7j
Content-Type: application/pgp-signature; name="signature.asc"
Content-Description: Digital signature

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1

iEYEARECAAYFAlhi44oACgkQMOfwapXb+vJBiQCeK1PRrK0Ck9bhMIYF7bQIIpSA
8ygAoITV5c7YJnNaZnpld2qV+QRGdR+B
=hImY
-----END PGP SIGNATURE-----

--nFreZHaLTZJo0R7j--