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

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Date:	Fri, 13 Apr 2012 11:33:28 -0700
From:	Sheng Yang <sheng@...ker.org>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	xen-devel@...ts.xensource.com,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Tim Deegan <tim@....org>, Jan Beulich <jbeulich@...e.com>
Subject: Re: [PATCH] xen: always set the sched clock as unstable

On Fri, Apr 13, 2012 at 11:20 AM, David Vrabel <david.vrabel@...rix.com> wrote:
>
> From: David Vrabel <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>
> ---
>  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
>

(Sorry for duplicate posts, gmail is really not a ideal client for
lkml - though exchange is worse...)

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".

For the other context, please refer to the discussion at xen-devel.

http://lists.xen.org/archives/html/xen-devel/2012-04/msg00888.html
http://lists.xen.org/archives/html/xen-devel/2012-04/msg00969.html

--
regards
Yang, Sheng
--
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