linux-kernel - Re: [PATCH 2/2] Revert "x86/hpet: Reduce HPET counter read contention"

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Date:   Mon, 18 Mar 2019 16:44:47 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Srinivas Eeda <srinivas.eeda@...cle.com>, x86@...nel.org
Subject: Re: [PATCH 2/2] Revert "x86/hpet: Reduce HPET counter read
 contention"


On 2019/3/15 22:17, Waiman Long wrote:
> On 03/15/2019 05:25 AM, Peter Zijlstra wrote:
>> On Thu, Mar 14, 2019 at 04:42:12PM +0800, Zhenzhong Duan wrote:
>>> This reverts commit f99fd22e4d4bc84880a8a3117311bbf0e3a6a9dc.
>>>
>>> It's unnecessory after commit "acpi_pm: Fix bootup softlockup due to PMTMR
>>> counter read contention", the simple HPET access code could be restored.
>>>
>>> On a general system with good TSC, TSC is the final default clocksource.
>>> So the potential performce loss is only at bootup stage before TSC
>>> replacing HPET, we didn't observe obvious delay of bootup.
>> The timeline here is:
>>
>>   - Len took out SKX from native_calibrate_tsc
>>     b51120309348 ("x86/tsc: Fix erroneous TSC rate on Skylake Xeon")
>>
>>     This causes the TSC to run through the calibration code, which
>>     completes _after_ SMP bringup.
>>
>>   - This then caused HPET to be used during SMP bringup, which resulted
>>     in Waiman doing the patch you now propose removing.
>>
>>     Because large (multi-socket) SKX machines would barely boot.
>>
>>     f99fd22e4d4b ("x86/hpet: Reduce HPET counter read contention")
>>
>>   - Now, I figured that was all crazy to begin with, and introduced
>>     clocksource_tsc_early, such that we can run at the guestimate TSC
>>     frequency until we've completed calibration and then swap to the real
>>     TSC clocksource.
>>
>>     aa83c45762a2 ("x86/tsc: Introduce early tsc clocksource")
>>     (and assorted fixes)
>>
>> This means that we now only use HPET for a very short time in early
>> boot, _IFF_ TSC is stable.
>>
>> Now, given the amount of wreckage we still see with TSC, I'm very
>> reluctant to revert this patch. Because the moment TSC goes out the
>> window, we're back on HPET, and this patch does make a huge difference.
>>
>> Yes, its sad, gross and nasty... but the same is true for TSC still being
>> a trainwreck.
>>
>> So NAK.
> I concur. In the uncontended case, the overhead is mostly just the
> additional cmpxchg instruction for acquiring the spinlock. Even then, it
> isn't significant when compared with the time needed to actually read
> from the HPET. Without that code, any fallback to HPET for whatever
> reason will likely see degradation in performance especially on systems
> with large number of CPUs.

Thank Peter and Waiman for reply.

I see, we still care the performance on a system with wreckage TSC.


So now we come back to the old question, do we care the softlockup

and the performance when pmtmr is chosed for whatever reason?

For which I had provide two different fixes:

https://lkml.org/lkml/2019/1/22/1172

and

https://lkml.org/lkml/2019/3/15/101

-- 
Thanks
Zhenzhong