Re: [LKP] Re: [clocksource] 6c52b5f3cf: stress-ng.opcode.ops_per_sec -14.4% regression

Paul E. McKenney
2021-04-22T14:24:58Z

On Thu, Apr 22, 2021 at 03:41:26PM +0800, Feng Tang wrote:
> Hi Paul,
>
> On Thu, Apr 22, 2021 at 02:58:27PM +0800, Xing Zhengjun wrote:
> >
> >
> > On 4/21/2021 9:42 PM, Paul E. McKenney wrote:
> > >On Wed, Apr 21, 2021 at 02:07:19PM +0800, Xing, Zhengjun wrote:
> > >>
> > >>On 4/20/2021 10:05 PM, Paul E. McKenney wrote:
> > >>>On Tue, Apr 20, 2021 at 06:43:31AM -0700, Paul E. McKenney wrote:
> > >>>>On Tue, Apr 20, 2021 at 02:49:34PM +0800, kernel test robot wrote:
> > >>>>>Greeting,
> > >>>>>
> > >>>>>FYI, we noticed a -14.4% regression of stress-ng.opcode.ops_per_sec due to commit:
> > >>>>>
> > >>>>>
> > >>>>>commit: 6c52b5f3cfefd6e429efc4413fd25e3c394e959f ("clocksource: Reduce WATCHDOG_THRESHOLD")
> > >>>>>https://git.kernel.org/cgit/linux/kernel/git/paulmck/linux-rcu.git dev.2021.04.13a
> > >>>>>
> > >>>>>
> > >>>>>in testcase: stress-ng
> > >>>>>on test machine: 96 threads Intel(R) Xeon(R) Gold 6252 CPU @ 2.10GHz with 192G memory
> > >>>>>with following parameters:
> > >>>>>
> > >>>>> nr_threads: 10%
> > >>>>> disk: 1HDD
> > >>>>> testtime: 60s
> > >>>>> fs: ext4
> > >>>>> class: os
> > >>>>> test: opcode
> > >>>>> cpufreq_governor: performance
> > >>>>> ucode: 0x5003006
> > >>>>Hmmm... I will try a less-aggressive reduction. Thank you for testing!
> > >>>But wait... This code is only running twice per second. It is very
> > >>>hard to believe that a clock-read retry twice per second is worth 2% of
> > >>>performance, let alone 14.4%.
> > >>>
> > >>>Is something else perhaps going on here?
> > >>>
> > >>>For example, did this run enable any of the new diagnositic clocksource.*
> > >>>kernel parameters?
> > >>>
> > >>> Thanx, Paul
> > >>I attached the kernel log, the following logs are related with the
> > >>clocksource.
> > >>[    3.453206] clocksource: timekeeping watchdog on CPU1: Marking
> > >>clocksource 'tsc-early' as unstable because the skew is too large:
> > >>[    3.455197] clocksource:                       'hpet' wd_now: 288fcc0
> > >>wd_last: 1a8b333 mask: ffffffff
> > >>[    3.455199] clocksource:                       'tsc-early' cs_now:
> > >>1def309ebfdee cs_last: 1def2bd70d92c mask: ffffffffffffffff
> > >>[    3.455201] clocksource:                       No current clocksource.
> > >>[    3.457197] tsc: Marking TSC unstable due to clocksource watchdog
> > >>
> > >>6c52b5f3cf reduced WATCHDOG_THRESHOLD, then in clocksource_watchdog, the
> > >>warning logs are print, the TSC is marked as unstable.
> > >>/* Check the deviation from the watchdog clocksource. */
> > >Aha, so this system really does have an unstable TSC! Which means that
> > >the patch is operating as designed.
> > >
> > >Or are you saying that this is a false positive?
> > >
> > > Thanx, Paul
> >
> > It happened during boot and before TSC calibration
> > (tsc_refine_calibration_work()), so on some machines "abs(cs_nsec - wd_nsec)
> > > WATCHDOG_THRESHOLD", WATCHDOG_THRESHOLD is set too small at that time.
> > After TSC calibrated, abs(cs_nsec - wd_nsec) should be very small,
> > WATCHDOG_THRESHOLD for here is ok. So I suggest increasing the
> > WATCHDOG_THRESHOLD before TSC calibration, for example, the clocks be skewed
> > by more than 1% to be marked unstable.

This is common code, so we do need an architecture-independent way to
handle this.

> As Zhengjun measuered, this is a Cascade Lake platform, and it has 2
> times calibration of tsc, the first one of early quick calibration gives
> 2100 MHz, while the later accurate calibration gives 2095 MHz, so there
> is about 2.5/1000 deviation for the first number, which just exceeds the
> 1/1000 threshold you set :)

Even my 2/1000 initial try would have caused this, then. ;-)

But even 1/1000 deviation would cause any number of applications some
severe heartburn, so I am not at all happy with the thought of globally
increasing to (say) 3/1000.

> Following is the tsc freq info from kernel log
>
> [ 0.000000] DMI: Intel Corporation S2600WFT/S2600WFT, BIOS SE5C620.86B.02.01.0008.031920191559 03/19/2019
> [ 0.000000] tsc: Detected 2100.000 MHz processor
> ...
> [ 13.859982] tsc: Refined TSC clocksource calibration: 2095.077 MHz

So what are our options?

1. Clear CLOCK_SOURCE_MUST_VERIFY from tsc-early.

2. #1, but add tsc-early into the watchdog list and set
CLOCK_SOURCE_MUST_VERIFY once it is better calibrated.

3. Add a field to struct clocksource that, if non-zero, gives
the maximum drift in nanoseconds per half second (AKA
WATCHDOG_INTERVAL). If zero, the WATCHDOG_MAX_SKEW value
is used. Set this to (say) 150,000ns for tsc-early.

4. As noted earlier, increase WATCHDOG_MAX_SKEW to 150 microseconds,
which again is not a good approach given the real-world needs
of real-world applications.

5. Your ideas here.

All in all, I am glad that I made the patch that decreases
WATCHDOG_MAX_SKEW be separate and at the end of the series. ;-)

Thanx, Paul