linux-kernel - Re: [PATCH v13 13/18] x86/tsc: calibrate tsc only once

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Date:   Mon, 16 Jul 2018 17:32:20 +0800
From:   Dou Liyang <douly.fnst@...fujitsu.com>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
CC:     Steven Sistare <steven.sistare@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        <linux@...linux.org.uk>, <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        John Stultz <john.stultz@...aro.org>, <sboyd@...eaurora.org>,
        <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        <mingo@...hat.com>, <tglx@...utronix.de>, <hpa@...or.com>,
        <peterz@...radead.org>, <prarit@...hat.com>, <feng.tang@...el.com>,
        Petr Mladek <pmladek@...e.com>, <gnomes@...rguk.ukuu.org.uk>,
        <linux-s390@...r.kernel.org>, <boris.ostrovsky@...cle.com>,
        <jgross@...e.com>
Subject: Re: [PATCH v13 13/18] x86/tsc: calibrate tsc only once



At 07/13/2018 07:30 PM, Pavel Tatashin wrote:
> On Fri, Jul 13, 2018 at 3:24 AM Dou Liyang <douly.fnst@...fujitsu.com> wrote:
>>
>>
>> At 07/12/2018 08:04 AM, Pavel Tatashin wrote:
>>> During boot tsc is calibrated twice: once in tsc_early_delay_calibrate(),
>>> and the second time in tsc_init().
>>>
>>> Rename tsc_early_delay_calibrate() to tsc_early_init(), and rework it so
>>> the calibration is done only early, and make tsc_init() to use the values
>>> already determined in tsc_early_init().
>>>
>>> Sometimes it is not possible to determine tsc early, as the subsystem that
>>> is required is not yet initialized, in such case try again later in
>>> tsc_init().
>>>
>>> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
>>> Signed-off-by: Pavel Tatashin <pasha.tatashin@...cle.com>
>>
>> Hi Pavel,
>>
>> Aha, a complex solution for a simple problem! ;-) And I did find any
>> benefits of doing that. did I miss something?
> 
> Hi Dou,
> 
> I had this in previous version:  init early, and unconditionally
> re-init later (which required to resync sched clocks for continuity,
> and check for some other corner cases). Thomas did not like the idea,
> as it is less deterministic: it leads for code to work by accident,
> where we might get one tsc frequency early and another later, and so
> on. The request was to initialize only once, and if that fails do it
> again later. This way, if early initialization is broken, we will know
> and fix it.
> 

Hi Pavel,

Yes, right, I have seen the purpose in v12.

>>
>> As the cpu_khz and tsc_khz are global variables and the tsc_khz may
>> be reset to cpu_khz. How about the following patch.
> 
> Could you please explain where you think this patch can be applied,
> and what it fixes?
> 

This patch is just an simple alternative to realize what you want in
your patch. your patch is also good but complex, and need some scrub.
eg:
   - Is it suitable to using the WARN_ON()
   - the name of determine_cpu_tsc_frequncies() function,
    s/frequncies/frequencies/. BTW, How about tsc_calibrate_frequencies()
   - ...

BTW, before this patch, seems we need make sure xen should work well, I
can investigate and try to test if we can also move the pagetable_init()
to the front of tsc_early_init() for you.

Thanks,
	dou

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index da1dbd99cb6e..74cb16d89e25 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1197,12 +1197,12 @@ void __init setup_arch(char **cmdline_p)

         memblock_find_dma_reserve();

+       x86_init.paging.pagetable_init();
+
         tsc_early_delay_calibrate();
         if (!early_xdbc_setup_hardware())
                 early_xdbc_register_console();

-       x86_init.paging.pagetable_init();
-
         kasan_init();

         /*

> Thank you,
> Pavel
> 
>>
>> Thanks,
>>          dou
>> ------------------------8<-----------------------------------
>>
>> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
>> index 74392d9d51e0..e54fa1037d45 100644
>> --- a/arch/x86/kernel/tsc.c
>> +++ b/arch/x86/kernel/tsc.c
>> @@ -1370,8 +1370,10 @@ void __init tsc_init(void)
>>                   return;
>>           }
>>
>> -       cpu_khz = x86_platform.calibrate_cpu();
>> -       tsc_khz = x86_platform.calibrate_tsc();
>> +       if (!tsc_khz) {
>> +               cpu_khz = x86_platform.calibrate_cpu();
>> +               tsc_khz = x86_platform.calibrate_tsc();
>> +       }
>>
>>           /*
>>            * Trust non-zero tsc_khz as authorative,
> 
> 
>