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Re: [Xen-devel] [PATCH] xen: always set the sched clock as unstable

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

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

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
 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)

+       sched_clock_stable = 0;


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.


Yang, Sheng

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