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

Feng Tang
2021-04-22T07:41:37Z

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.

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 :)

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

Thanks,
Feng