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

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Date:   Thu, 10 Jan 2019 18:10:31 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.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 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.

-boris