From: Matt Sealey <matt@genesi-usa.com>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: John Stultz <john.stultz@linaro.org>,
Arnd Bergmann <arnd@arndb.de>,
Linux ARM Kernel ML <linux-arm-kernel@lists.infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>
Subject: Re: One of these things (CONFIG_HZ) is not like the others..
Date: Mon, 21 Jan 2013 17:30:31 -0600 [thread overview]
Message-ID: <CAKGA1b=wDE9ujby_gcoHJudsG=JbWDdNRo+thCe8BhoJLi+B5Q@mail.gmail.com> (raw)
In-Reply-To: <20130121231318.GC23505@n2100.arm.linux.org.uk>
On Mon, Jan 21, 2013 at 5:13 PM, Russell King - ARM Linux
<linux@arm.linux.org.uk> wrote:
> On Mon, Jan 21, 2013 at 04:54:31PM -0600, Matt Sealey wrote:
>> Hmm, I think it might be appreciated for people looking at this stuff
>> (same as I stumbled into it) for a little comment on WHY the default
>> is 200. That way you don't wonder even if you know why EBSA110 has a
>> HZ=200 default, why Exynos is lumped in there too (to reduce the
>> number of interrupts firing?
>
> Err, _reduce_ ?
>
> Can you please explain why changing HZ from 100 to 200 is a reduction?
We were talking about HZ=1000 at the time, sorry...
>> Maybe the Exynos timer interrupt is kind
>> of a horrid core NMI kind of thing and it's desirable for it not to be
>> every millisecond,
>
> Huh? HZ=100 is centisecond intervals...
See above..
> I think you're understanding is just waaaayyyyy off. That default is
> there because that is the _architecture_ _default_ and there _has_ to
> be a default. No, including kernel/Kconfig.hz won't give us any kind
> of non-specified default because, as I've already said in one of my
> other mails, you can't supplement Kconfig symbol definitions by
> declaring it multiple times.
Okay, so the real
>> I know where the 60Hz clocksource might come from, the old Amiga
>> platforms have one based on the PSU frequency (50Hz in Europe, 60Hz
>> US/Japan). Even a 60Hz clocksource is useful though (on the Amiga, at
>> least, it is precisely the vsync clock for synchronizing your display
>> output on TV-out, which makes it completely useful for the framebuffer
>> driver), but.. you just won't expect to assign it as sched_clock or
>> your delay timer. And if anyone does I'd expect they'd know full well
>> it'd not run so well.
>
> Except in the UK where it'd be 50Hz for the TV out. (Lengthy irrelevant
> explanation why this is so for UK cut.)
Read again: "50Hz in Europe".
Australia too. I'm British and I used to have more EU-manufactured
Amigas than I knew what to do with.. so.. just like your NTP story, I
definitely know this already.
>> >From that description, we are booting with standard HZ on ARM, and the
>> core sched_clock (as in we can call setup_sched_clock)
>> and/or/both/optionally using a real delay_timer switch to HRT mode if
>> we have the right equipment available in the kernel and at runtime on
>> the SoC.. but the process scheduler isn't compiled with the means to
>> actually take advantage of us being in HRT mode?
>
> Don't mix sched_clock() into this; it has nothing to do with HZ at all.
> You're confusing your apples with your oranges.
Okay..
>> A simple BUILD_BUG_ON and a BUG_ON right after each other in the
>> appropriate clocksource driver solves that.. if there's an insistence
>> on having at least some rope, we can put them in a field and tell them
>> they have to use the moon to actually hang themselves...
>
> No it doesn't - it introduces a whole load of new ways to make the
> kernel build or boot fail for pointless reasons - more failures, more
> regressions.
>
> No thank you.
But it would effectively stop users drinking kool-aid.. if you set
your HZ to something stupid, you don't even get a kernel to build, and
certainly don't get to boot past the first 40 lines of boot messages..
I think most people would rather a build error, or a runtime
unmistakable, unmissable warning than a subtle and almost
imperceptible skew in NTP synchronization, to use your example.
--
Matt Sealey <matt@genesi-usa.com>
Product Development Analyst, Genesi USA, Inc.
next prev parent reply other threads:[~2013-01-21 23:30 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-01-21 20:01 One of these things (CONFIG_HZ) is not like the others Matt Sealey
2013-01-21 20:41 ` Arnd Bergmann
2013-01-21 21:00 ` John Stultz
2013-01-21 21:12 ` Russell King - ARM Linux
2013-01-21 22:18 ` John Stultz
2013-01-21 22:44 ` Russell King - ARM Linux
2013-01-22 8:27 ` Arnd Bergmann
2013-01-21 22:20 ` Matt Sealey
2013-01-21 22:42 ` Russell King - ARM Linux
2013-01-21 23:23 ` Matt Sealey
2013-01-21 23:49 ` Russell King - ARM Linux
2013-01-22 0:09 ` Matt Sealey
2013-01-22 0:26 ` Matt Sealey
2013-01-21 21:14 ` Matt Sealey
2013-01-21 22:36 ` John Stultz
2013-01-21 22:49 ` Russell King - ARM Linux
2013-01-21 22:54 ` Matt Sealey
2013-01-21 23:13 ` Russell King - ARM Linux
2013-01-21 23:30 ` Matt Sealey [this message]
2013-01-22 0:02 ` Russell King - ARM Linux
2013-01-22 0:38 ` John Stultz
2013-01-22 0:51 ` John Stultz
2013-01-22 1:06 ` Matt Sealey
2013-01-22 1:18 ` Russell King - ARM Linux
2013-01-22 1:56 ` Matt Sealey
2013-01-22 1:31 ` John Stultz
2013-01-22 2:10 ` Matt Sealey
2013-01-31 21:31 ` Thomas Gleixner
2013-01-21 21:02 ` Matt Sealey
2013-01-21 22:30 ` Arnd Bergmann
2013-01-21 22:45 ` Russell King - ARM Linux
2013-01-21 23:01 ` Matt Sealey
2013-01-21 21:03 ` Russell King - ARM Linux
2013-01-21 23:23 ` Tony Lindgren
2013-01-22 6:23 ` Santosh Shilimkar
2013-01-22 9:31 ` Arnd Bergmann
2013-01-22 10:14 ` Santosh Shilimkar
2013-01-22 14:51 ` Russell King - ARM Linux
2013-01-22 15:05 ` Santosh Shilimkar
2013-01-28 6:08 ` Santosh Shilimkar
2013-01-29 0:01 ` John Stultz
2013-01-29 6:43 ` Santosh Shilimkar
2013-01-29 10:06 ` Russell King - ARM Linux
2013-01-29 18:43 ` John Stultz
2013-01-22 17:31 ` Arnd Bergmann
2013-01-22 18:59 ` John Stultz
2013-01-22 21:52 ` Tony Lindgren
2013-01-23 5:18 ` Santosh Shilimkar
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