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From: John Stultz <john.stultz@linaro.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: lkml <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Prarit Bhargava <prarit@redhat.com>,
	Richard Cochran <richardcochran@gmail.com>
Subject: Re: [PATCH 19/21] clocksource: Improve comment explaining clocks_calc_max_nsecs()'s 50% safety margin
Date: Thu, 2 Apr 2015 10:30:18 -0700	[thread overview]
Message-ID: <CALAqxLWzy5uTy_YZ8D5As5wGMPeQ=TZrpDes8SXVj4ws8bUuzQ@mail.gmail.com> (raw)
In-Reply-To: <20150402085011.GJ21418@twins.programming.kicks-ass.net>

On Thu, Apr 2, 2015 at 1:50 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, Apr 01, 2015 at 08:34:39PM -0700, John Stultz wrote:
>> Ingo noted that the description of clocks_calc_max_nsecs()'s
>> 50% safety margin was somewhat circular. So this patch tries
>> to improve the comment to better explain what we mean by the
>> 50% safety margin and why we need it.
>>
>> Cc: Ingo Molnar <mingo@kernel.org>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Prarit Bhargava <prarit@redhat.com>
>> Cc: Richard Cochran <richardcochran@gmail.com>
>> Signed-off-by: John Stultz <john.stultz@linaro.org>
>> ---
>>  kernel/time/clocksource.c | 7 +++++--
>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
>> index c3be3c7..15facb1 100644
>> --- a/kernel/time/clocksource.c
>> +++ b/kernel/time/clocksource.c
>> @@ -472,8 +472,11 @@ static u32 clocksource_max_adjustment(struct clocksource *cs)
>>   * @max_cyc: maximum cycle value before potential overflow (does not include
>>   *           any safety margin)
>>   *
>> - * NOTE: This function includes a safety margin of 50%, so that bad clock values
>> - * can be detected.
>> + * NOTE: This function includes a safety margin of 50%, in other words, we
>> + * return half the number of nanoseconds the hardware counter can technically
>> + * cover. This is done so that we can potentially detect problems caused by
>> + * delayed timers or bad hardware, which might result in time intervals that
>> + * are larger then what the math used can handle without overflows.
>>   */
>>  u64 clocks_calc_max_nsecs(u32 mult, u32 shift, u32 maxadj, u64 mask, u64 *max_cyc)
>>  {
>
> Should we make a further note that the tk_fast things rely on this
> slack since they're not strongly serialized against this? That is, they
> can end up using an older cycle_last value and therefore end up with a
> larger delta than other code.

Though, even with the tk_fast bits, we expect the update to happen
regularly, its just that for the benefit of lock-free access we are ok
with the possible slight inconsistencies (in the mono clock) that
could happen if we use a slightly stale value mid-update. So I don't
think the tk_fast bits are actually relying on the slack any more then
the normal timekeeping code relies on the slack to handle slight
delays in processing the updates.  If we deal with time deltas large
enough to cause overflows, or time intervals larger then the hardware
can represent, we're sunk in either case. This 50% margin just makes
it easier to catch unexpected delays or issues.

thanks
-john

  reply	other threads:[~2015-04-02 17:30 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-04-02  3:34 [PATCH 00/21] 4.1 time and rtc changes for tip/timers/core John Stultz
2015-04-02  3:34 ` [PATCH 01/21] time: Add y2038 safe read_boot_clock64() John Stultz
2015-04-03  8:15   ` [tip:timers/core] " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 02/21] time: Add y2038 safe read_persistent_clock64() John Stultz
2015-04-03  8:15   ` [tip:timers/core] " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 03/21] time: Add y2038 safe update_persistent_clock64() John Stultz
2015-04-03  8:16   ` [tip:timers/core] time: Add y2038 safe update_persistent_clock64( ) tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 04/21] ARM: OMAP: 32k counter: Provide y2038-safe omap_read_persistent_clock() replacement John Stultz
2015-04-03  8:16   ` [tip:timers/core] " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 05/21] ARM: tegra: clock: Provide y2038-safe tegra_read_persistent_clock() replacement John Stultz
2015-04-03  8:16   ` [tip:timers/core] clocksource/drivers/tegra: " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 06/21] ARM: time: Provide read_boot_clock64() and read_persistent_clock64() John Stultz
2015-04-03  8:17   ` [tip:timers/core] ARM, clocksource/drivers: Provide read_boot_clock64() and read_persistent_clock64() and use them tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 07/21] rtc: Provide y2038 safe rtc_class_ops.set_mmss() replacement John Stultz
2015-04-03  8:17   ` [tip:timers/core] drivers/rtc: " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 08/21] rtc/test: Update driver to address y2038/y2106 issues John Stultz
2015-04-03  8:17   ` [tip:timers/core] drivers/rtc/test: " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 09/21] rtc/ab3100: " John Stultz
2015-04-03  8:17   ` [tip:timers/core] drivers/rtc/ab3100: " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 10/21] rtc/mc13xxx: " John Stultz
2015-04-03  8:18   ` [tip:timers/core] drivers/rtc/mc13xxx: " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 11/21] rtc/mxc: Modify rtc_update_alarm() not to touch the alarm time John Stultz
2015-04-03  8:18   ` [tip:timers/core] drivers/rtc/mxc: " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 12/21] rtc/mxc: Convert get_alarm_or_time()/set_alarm_or_time() to use time64_t John Stultz
2015-04-03  8:18   ` [tip:timers/core] drivers/rtc/mxc: Convert get_alarm_or_time()/ set_alarm_or_time() " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 13/21] rtc/mxc: Update driver to address y2038/y2106 issues John Stultz
2015-04-03  8:19   ` [tip:timers/core] drivers/rtc/mxc: Update driver to address y2038 /y2106 issues tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 14/21] alpha: rtc: Change to use rtc_class_ops's set_mmss64() John Stultz
2015-04-03  8:19   ` [tip:timers/core] alpha, rtc: Change to use rtc_class_ops' s set_mmss64() tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 15/21] time: Don't build timekeeping_inject_sleeptime64() if no one uses it John Stultz
2015-04-03  8:19   ` [tip:timers/core] time: Don' t " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 16/21] rtc: Remove redundant rtc_valid_tm() from rtc_resume() John Stultz
2015-04-03  8:19   ` [tip:timers/core] drivers/rtc: " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 17/21] time: Fix a bug in timekeeping_suspend() with no persistent clock John Stultz
2015-04-03  6:16   ` Ingo Molnar
2015-04-03  8:20   ` [tip:timers/core] " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 18/21] time: rtc: Don't bother into rtc_resume() for the nonstop clocksource John Stultz
2015-04-03  8:20   ` [tip:timers/core] time, drivers/rtc: Don't bother with rtc_resume () " tip-bot for Xunlei Pang
2015-04-02  3:34 ` [PATCH 19/21] clocksource: Improve comment explaining clocks_calc_max_nsecs()'s 50% safety margin John Stultz
2015-04-02  8:50   ` Peter Zijlstra
2015-04-02 17:30     ` John Stultz [this message]
2015-04-02 18:34       ` Peter Zijlstra
2015-04-02 18:41         ` John Stultz
2015-04-02 18:43           ` Peter Zijlstra
2015-04-02 18:50             ` John Stultz
2015-04-02 19:04               ` Peter Zijlstra
2015-04-03  8:20   ` [tip:timers/core] " tip-bot for John Stultz
2015-04-02  3:34 ` [PATCH 20/21] timekeeping: Change timekeeping_check_update() to take a tk_read_base John Stultz
2015-04-02  3:34 ` [PATCH 21/21] time: Rework debugging variables so they aren't global John Stultz
2015-04-02  7:47   ` Peter Zijlstra
2015-04-02  7:51     ` Ingo Molnar
2015-04-02  8:36       ` Peter Zijlstra
2015-04-02  8:41         ` Peter Zijlstra
2015-04-02 17:32       ` John Stultz
2015-04-03  6:21         ` Ingo Molnar

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