linux-kernel - Re: [PATCH] xen: always set the sched clock as unstable

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Message-ID: <4F8872D7.7050507@citrix.com>
Date:	Fri, 13 Apr 2012 19:39:19 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Sheng Yang <sheng@...ker.org>
CC:	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Tim (Xen.org)" <tim@....org>, Jan Beulich <jbeulich@...e.com>
Subject: Re: [PATCH] xen: always set the sched clock as unstable

On 13/04/12 19:31, Sheng Yang wrote:
> On Fri, Apr 13, 2012 at 11:20 AM, David Vrabel <david.vrabel@...rix.com
> <mailto:david.vrabel@...rix.com>> wrote:
> 
>     From: David Vrabel <david.vrabel@...rix.com
>     <mailto:david.vrabel@...rix.com>>
> 
>     The sched clock was considered stable based on the capabilities of the
>     underlying hardware.  This does not make sense for Xen PV guests as:
>     a) the hardware TSC is not used directly as the clock source; and b)
>     guests may migrate to hosts with different hardware capabilities.
> 
>     It is not clear to me whether the Xen clock source is supposed to be
>     stable and whether it should be stable across migration.  For a clock
>     source to be stable it must be: a) monotonic; c) synchronized across
>     CPUs; and c) constant rate.
> 
>     There have also been reports of systems with apparently unstable
>     clocks where clearing sched_clock_stable has fixed problems with
>     migrated VMs hanging.
> 
>     So, always set the sched clock as unstable when using the Xen clock
>     source.
> 
>     Signed-off-by: David Vrabel <david.vrabel@...rix.com
>     <mailto:david.vrabel@...rix.com>>
>     ---
>      arch/x86/xen/time.c |    1 +
>      1 files changed, 1 insertions(+), 0 deletions(-)
> 
>     diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
>     index 0296a95..8469b5a 100644
>     --- a/arch/x86/xen/time.c
>     +++ b/arch/x86/xen/time.c
>     @@ -473,6 +473,7 @@ static void __init xen_time_init(void)
>            do_settimeofday(&tp);
> 
>            setup_force_cpu_cap(X86_FEATURE_TSC);
>     +       sched_clock_stable = 0;
> 
>            xen_setup_runstate_info(cpu);
>            xen_setup_timer(cpu);
>     --
>     1.7.2.5
> 
> 
> I really prefer to hide nonstop tsc feature from Xen side, rather than
> let Linux kernel to have a condition that "Nonstop TSC feature existed,
> but sched_clock_stable=0".

I disagree.  The decision on whether the clock is stable or not should
be in the Xen clock source code.

David
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