Re: [PATCH] acpi_pm: Reduce PMTMR counter read contention

[Zhenzhong Duan <zhenzhong.duan@oracle.com>]

On 2019/1/30 16:06, Thomas Gleixner wrote:
> On Tue, 22 Jan 2019, Zhenzhong Duan wrote:
> 
>> On a large system with many CPUs, using PMTMR as the clock source can
>> have a significant impact on the overall system performance because
>> of the following reasons:
>>   1) There is a single PMTMR counter shared by all the CPUs.
>>   2) PMTMR counter reading is a very slow operation.
>>
>> Using PMTMR as the default clock source may happen when, for example,
>> the TSC clock calibration exceeds the allowable tolerance and HPET
>> disabled by nohpet on kernel command line. Sometimes the performance
> 
> The question is why would anyone disable HPET on a larger machine when the
> TSC is wreckaged?

There may be broken hardware where TSC is wreckaged.
On our instances(X8-8/X7-8), TSC isn't wreckaged. Sometimes we are lucky 
to pass the bootup stage, then TSC is the final default clocksource. See 
log:
[    0.000000] clocksource: refined-jiffies: mask: 0xffffffff 
max_cycles: 0xffffffff, max_idle_ns: 1910969940391419 ns
[   13.963224] clocksource: jiffies: mask: 0xffffffff max_cycles: 
0xffffffff, max_idle_ns: 1911260446275000 ns
[   19.903175] clocksource: Switched to clocksource refined-jiffies
[   20.190467] clocksource: acpi_pm: mask: 0xffffff max_cycles: 
0xffffff, max_idle_ns: 2085701024 ns
[   20.201634] clocksource: Switched to clocksource acpi_pm
[   39.082577] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 
0x2113ba2fe3c, max_idle_ns: 440795266816 ns
[   39.138781] clocksource: Switched to clocksource tsc

When we are unlucky, logs:
[    0.000000] clocksource: refined-jiffies: mask: 0xffffffff 
max_cycles: 0xffffffff, max_idle_ns: 1910969940391419 ns
[   19.905741] clocksource: Switched to clocksource refined-jiffies
[   20.181521] clocksource: acpi_pm: mask: 0xffffff max_cycles: 
0xffffff, max_idle_ns: 2085701024 ns
[   44.273786] watchdog: BUG: soft lockup - CPU#48 stuck for 23s! 
[swapper/48:0]
[   44.279992] watchdog: BUG: soft lockup - CPU#49 stuck for 23s! 
[migration/49:307]

So we paniced when acpi_pm is initializing and is chosed as default 
clocksource temporarily, it paniced just because we add nohpet parameter.
> 
> I'm not against the change per se, but I really want to understand why we
> need all the complexity for something which should never be used in a real
> world deployment.

Hmm, it's a strong word of "never be used". Customers may happen to use 
nohpet(sanity test?) and report bug to us. Sometimes they does report a 
bug that reproduce with their customed config. There may also be BIOS 
setting HPET disabled.

Thanks
Zhenzhong