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Reply-To: zhenzhong.duan@oracle.com
Subject: Re: [PATCH] acpi_pm: Reduce PMTMR counter read contention
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Waiman Long <longman@redhat.com>,
        Srinivas Eeda <srinivas.eeda@oracle.com>, kin.cho@oracle.com
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 <alpine.DEB.2.21.1901311525360.8200@nanos.tec.linutronix.de>
From:   Zhenzhong Duan <zhenzhong.duan@oracle.com>
Organization: Oracle
Message-ID: <853e8cf6-aba9-0200-8e39-e362848399ba@oracle.com>
Date:   Sat, 2 Feb 2019 10:52:36 +0800
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On 2019/1/31 22:26, Thomas Gleixner wrote:
> On Thu, 31 Jan 2019, Zhenzhong Duan wrote:
> 
>> 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.
> 
> I know that.
> 
>>> 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.
> 
> And because the customer MAY do completely nonsensical things (and there
> are a lot more than the HPET) the kernel has to handle all of them?

Ok, then. I don't have more suggestion to convince you. I just think of 
a simple fix as below. I think it will work for both hpet and pmtmr. We 
will test it when the env is available.

--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1353,6 +1353,7 @@ static int change_clocksource(void *data)

         write_seqcount_end(&tk_core.seq);
         raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
+       tick_clock_notify();

         return 0;
  }
@@ -1371,7 +1372,6 @@ int timekeeping_notify(struct clocksource *clock)
         if (tk->tkr_mono.clock == clock)
                 return 0;
         stop_machine(change_clocksource, clock, NULL);
-       tick_clock_notify();
         return tk->tkr_mono.clock == clock ? 0 : -1;
  }


Thanks
Zhenzhong