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From: Richard Cochran <richardcochran@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: John Stultz <john.stultz@linaro.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Dave Jones <davej@codemonkey.org.uk>,
	Thomas Gleixner <tglx@linutronix.de>,
	Prarit Bhargava <prarit@redhat.com>,
	Stephen Boyd <sboyd@codeaurora.org>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 06/10] time: Cap clocksource reads to the clocksource max_cycles value
Date: Mon, 12 Jan 2015 21:37:57 +0100	[thread overview]
Message-ID: <20150112203757.GC4233@localhost.localdomain> (raw)
In-Reply-To: <CA+55aFxwtAG243YL5xJwCy1tJOOONx_ZqYBFsJwwbaFTmKWfLg@mail.gmail.com>

On Tue, Jan 13, 2015 at 08:02:53AM +1300, Linus Torvalds wrote:
> Indeed. It's making things more robust in the face of _known_ issues.
> Even with a perfectly designed timer (which we so far have never
> seen), interrupts get delayed etc, so trying to stretch it to the
> limit of the timer is simply not a good idea. Quite the reverse.

So is this patch supposed to fix the case when a tick just missed the
range of the clock?  Or is this to deal with really broken sources?

> More importantly, if the timer is actually any good, the safety margin
> won't actually matter, since the timer cycle is so long that 50% of
> essentially infinite is still essentially infinite.
> 
> And if the timer isn't very good, then some slop for safety is just
> being robust.

Here "isn't very good" means that the clock rolls over too frequently.
Well, if you cannot be sure to sample the clock in time, then you
shouldn't use that clock source at all.

Thanks,
Richard

  reply	other threads:[~2015-01-12 20:38 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-01-10  0:34 [PATCH 00/10][RFC] Increased clocksource validation and cleanups John Stultz
2015-01-10  0:34 ` [PATCH 01/10] clocksource: Simplify clocks_calc_max_nsecs logic John Stultz
2015-01-10  0:34 ` [PATCH 02/10] clocksource: Simplify logic around clocksource wrapping saftey margins John Stultz
2015-01-10  2:03   ` Stephen Boyd
2015-01-10  0:34 ` [PATCH 03/10] clocksource: Remove clocksource_max_deferment() John Stultz
2015-01-11 11:47   ` Richard Cochran
2015-01-12 18:36     ` John Stultz
2015-01-12 20:16       ` Richard Cochran
2015-01-10  0:34 ` [PATCH 04/10] clocksource: Add max_cycles to clocksource structure John Stultz
2015-01-10  2:06   ` Stephen Boyd
2015-01-10  0:34 ` [PATCH 05/10] time: Add debugging checks to warn if we see delays John Stultz
2015-01-10  0:34 ` [PATCH 06/10] time: Cap clocksource reads to the clocksource max_cycles value John Stultz
2015-01-11 12:41   ` Richard Cochran
2015-01-12 18:54     ` John Stultz
2015-01-12 19:02       ` Linus Torvalds
2015-01-12 20:37         ` Richard Cochran [this message]
2015-01-12 20:30       ` Richard Cochran
2015-01-12 20:49         ` Richard Cochran
2015-01-13 11:11   ` Peter Zijlstra
2015-01-13 21:33     ` John Stultz
2015-01-13 22:51       ` Linus Torvalds
2015-01-14  9:35       ` Peter Zijlstra
2015-01-22 20:55         ` John Stultz
2015-01-10  0:34 ` [PATCH 07/10] time: Try to catch clocksource delta underflows John Stultz
2015-01-10  0:34 ` [PATCH 08/10] clocksource: Mostly kill clocksource_register() John Stultz
2015-01-10  0:34 ` [PATCH 09/10] sparc: Convert to using clocksource_register_hz() John Stultz
2015-01-10  0:34 ` [PATCH 10/10] clocksource: Add some debug info about clocksources being registered John Stultz
2015-01-10  2:02   ` Stephen Boyd
2015-01-22  0:51     ` John Stultz
2015-01-22 12:27       ` Prarit Bhargava
2015-01-11 11:41 ` [PATCH 00/10][RFC] Increased clocksource validation and cleanups Richard Cochran
2015-01-12 18:22   ` John Stultz
2015-01-12 20:45     ` Richard Cochran

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