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Subject: Re: [PATCH] xen: Fix x86 sched_clock() interface for xen
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
        x86@kernel.org
Cc:     sstabellini@kernel.org, hpa@zytor.com, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, hans@knorrie.org,
        stable@vger.kernel.org
References: <20190110100722.11216-1-jgross@suse.com>
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From:   Juergen Gross <jgross@suse.com>
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Date:   Fri, 11 Jan 2019 08:15:25 +0100
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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@oracle.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.

Sending out V2 soon.


Juergen