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Subject: Re: [PATCH] xen: Fix x86 sched_clock() interface for xen
To:     Juergen Gross <jgross@suse.com>, linux-kernel@vger.kernel.org,
        xen-devel@lists.xenproject.org, x86@kernel.org
Cc:     boris.ostrovsky@oracle.com, sstabellini@kernel.org, hpa@zytor.com,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        stable@vger.kernel.org
References: <20190110100722.11216-1-jgross@suse.com>
From:   Hans van Kranenburg <hans@knorrie.org>
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Message-ID: <9a48473d-2231-8e34-8ef7-bb9c7e2b3dbb@knorrie.org>
Date:   Thu, 10 Jan 2019 17:19:44 +0100
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On 1/10/19 11:07 AM, Juergen Gross wrote:
> Commit f94c8d11699759 ("sched/clock, x86/tsc: Rework the x86 'unstable'
> sched_clock() interface") broke Xen guest time handling across
> migration:
> 
> [  187.249951] Freezing user space processes ... (elapsed 0.001 seconds) done.
> [  187.251137] OOM killer disabled.
> [  187.251137] Freezing remaining freezable tasks ... (elapsed 0.001 seconds) done.
> [  187.252299] suspending xenstore...
> [  187.266987] xen:grant_table: Grant tables using version 1 layout
> [18446743811.706476] OOM killer enabled.
> [18446743811.706478] Restarting tasks ... done.
> [18446743811.720505] Setting capacity to 16777216

I tried this on top of 4.19.14 (together with "x86/mm: Fix guard hole
handling", which still doesn't seem to be in 4.19) and I can confirm the
fix:

Using a PV domU,

4.19 without patch:

[  646.199018] Freezing user space processes ... (elapsed 0.002 seconds)
done.
[  646.201305] OOM killer disabled.
[  646.201311] Freezing remaining freezable tasks ... (elapsed 0.001
seconds) done.
[  646.202699] suspending xenstore...
[  646.203005] xen:grant_table: Grant tables using version 1 layout
[18446200797.089367] OOM killer enabled.
[18446200797.089382] Restarting tasks ... done.


4.19 with patch, doing live migration movement between exactly the same
set of physical servers:

[   74.878062] Freezing user space processes ... (elapsed 0.002 seconds)
done.
[   74.880308] OOM killer disabled.
[   74.880314] Freezing remaining freezable tasks ... (elapsed 0.000
seconds) done.
[   74.881681] suspending xenstore...
[   74.887497] xen:grant_table: Grant tables using version 1 layout
[   74.942011] OOM killer enabled.
[   74.942025] Restarting tasks ... done.
[   74.947688] Setting capacity to 6291456
[   74.950833] Setting capacity to 10485760

Tested-by: Hans van Kranenburg <hans@knorrie.org>

Thanks!

> 
> Fix that by setting xen_sched_clock_offset at resume time to ensure a
> monotonic clock value.
> 
> Fixes: f94c8d11699759 ("sched/clock, x86/tsc: Rework the x86 'unstable' sched_clock() interface")
> Cc: <stable@vger.kernel.org> # 4.11
> Reported-by: Hans van Kranenburg <hans@knorrie.org>
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  arch/x86/xen/suspend.c |  4 ++++
>  arch/x86/xen/time.c    | 11 +++++++++++
>  arch/x86/xen/xen-ops.h |  2 ++
>  3 files changed, 17 insertions(+)
> 
> diff --git a/arch/x86/xen/suspend.c b/arch/x86/xen/suspend.c
> index 1d83152c761b..45fc9caf3880 100644
> --- a/arch/x86/xen/suspend.c
> +++ b/arch/x86/xen/suspend.c
> @@ -67,6 +67,8 @@ void xen_arch_resume(void)
>  {
>  	int cpu;
>  
> +	xen_clocksource_resume();
> +
>  	on_each_cpu(xen_vcpu_notify_restore, NULL, 1);
>  
>  	for_each_online_cpu(cpu)
> @@ -81,4 +83,6 @@ void xen_arch_suspend(void)
>  		xen_pmu_finish(cpu);
>  
>  	on_each_cpu(xen_vcpu_notify_suspend, NULL, 1);
> +
> +	xen_clocksource_suspend();
>  }
> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> index 72bf446c3fee..117ce958ffe6 100644
> --- a/arch/x86/xen/time.c
> +++ b/arch/x86/xen/time.c
> @@ -32,6 +32,7 @@
>  #define TIMER_SLOP	100000
>  
>  static u64 xen_sched_clock_offset __read_mostly;
> +static u64 xen_clock_value_saved;
>  
>  /* Get the TSC speed from Xen */
>  static unsigned long xen_tsc_khz(void)
> @@ -54,6 +55,16 @@ static u64 xen_clocksource_read(void)
>  	return ret;
>  }
>  
> +void xen_clocksource_suspend(void)
> +{
> +	xen_clock_value_saved = xen_clocksource_read() - xen_sched_clock_offset;
> +}
> +
> +void xen_clocksource_resume(void)
> +{
> +	xen_sched_clock_offset = xen_clocksource_read() - xen_clock_value_saved;
> +}
> +
>  static u64 xen_clocksource_get_cycles(struct clocksource *cs)
>  {
>  	return xen_clocksource_read();
> diff --git a/arch/x86/xen/xen-ops.h b/arch/x86/xen/xen-ops.h
> index 0e60bd918695..a17d3bdab6b8 100644
> --- a/arch/x86/xen/xen-ops.h
> +++ b/arch/x86/xen/xen-ops.h
> @@ -63,6 +63,8 @@ void __init xen_build_dynamic_phys_to_machine(void);
>  void __init xen_vmalloc_p2m_tree(void);
>  
>  void xen_init_irq_ops(void);
> +void xen_clocksource_suspend(void);
> +void xen_clocksource_resume(void);
>  void xen_setup_timer(int cpu);
>  void xen_setup_runstate_info(int cpu);
>  void xen_teardown_timer(int cpu);
>