linux-kernel - Re: [PATCH] xen: Fix x86 sched

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Date:   Fri, 11 Jan 2019 08:31:03 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        x86@...nel.org
Cc:     sstabellini@...nel.org, hpa@...or.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hans@...rrie.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] xen: Fix x86 sched_clock() interface for xen

On 11/01/2019 08:15, Juergen Gross wrote:
> On 11/01/2019 00:10, Boris Ostrovsky wrote:
>> On 1/10/19 12:17 PM, Boris Ostrovsky wrote:
>>> On 1/10/19 11:14 AM, Juergen Gross wrote:
>>>> On 10/01/2019 16:34, Boris Ostrovsky wrote:
>>>>> On 1/10/19 5:07 AM, Juergen Gross wrote:
>>>>>>  
>>>>>> +void xen_clocksource_suspend(void)
>>>>>> +{
>>>>>> +	xen_clock_value_saved = xen_clocksource_read() - xen_sched_clock_offset;
>>>>> xen_clock_value_saved = xen_sched_clock() maybe? 
>>>> I wanted xen_clocksource_suspend() and xen_clocksource_resume() to
>>>> be symmetrical to each other.
>>> OK.
>>>
>>> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>>
>>>> In case you are feeling strong about that, I'm not. :-) So in case
>>>> you insist on it I can change it. Or you can do so while committing.
>>
>>
>> I did some basic testing and noticed this (at loglevel=8):
>>
>> [   64.336488] Freezing user space processes ... (elapsed 0.001 seconds)
>> done.
>> [   64.337805] OOM killer disabled.
>> [   64.337814] Freezing remaining freezable tasks ... (elapsed 0.000
>> seconds) done.
>> [   64.339066] suspending xenstore...
>> [   85.888340] xen:grant_table: Grant tables using version 1 layout
>> [   64.359729] OOM killer enabled.
>> [   64.359736] Restarting tasks ... done.
>>
>>
>> Which made me think that perhaps we should do suspend/restore of the
>> clocksource as close as possible to HYPERVISOR_suspend() call, e.g. in
>> xen_arch_pre_suspend()/xen_arch_post_suspend():
>>
>> diff --git a/arch/x86/xen/suspend.c b/arch/x86/xen/suspend.c
>> index 45fc9caf3880..80ecba3fcc8c 100644
>> --- a/arch/x86/xen/suspend.c
>> +++ b/arch/x86/xen/suspend.c
>> @@ -22,6 +22,7 @@ static DEFINE_PER_CPU(u64, spec_ctrl);
>>  
>>  void xen_arch_pre_suspend(void)
>>  {
>> +       xen_clocksource_suspend();
>>         xen_save_time_memory_area();
>>  
>>         if (xen_pv_domain())
>> @@ -36,6 +37,7 @@ void xen_arch_post_suspend(int cancelled)
>>                 xen_hvm_post_suspend(cancelled);
>>  
>>         xen_restore_time_memory_area();
>> +       xen_clocksource_resume();
>>  }
>>  
>>  static void xen_vcpu_notify_restore(void *data)
>>
>>
>> This still has a window of incorrect clock value (you can see it, for
>> example, when xen_hvm_post_suspend() does pr_info("Xen HVM callback
>> vector for event delivery is enabled\n")), but it's smaller than before.
>> In particular, we will make time right before dpm_resume_start() call.
> 
> You are right, this is better.

In fact, I can just handle this entirely inside arch/x86/xen/time.c by
moving the required statements into xen_save_time_memory_area() and
xen_restore_time_memory_area().


Juergen