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

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Message-ID: <4F8C33E0.2080007@citrix.com>
Date:	Mon, 16 Apr 2012 15:59:44 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	xen-devel <xen-devel@...ts.xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Tim (Xen.org)" <tim@....org>, Sheng Yang <sheng@...ker.org>
Subject: Re: [PATCH] xen: always set the sched clock as unstable

On 16/04/12 12:32, Jan Beulich wrote:
>>>> On 13.04.12 at 20:20, 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.

Tim, Thomas, can you comment on the above paragraph?  Is it correct?

>> 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;
> 
> This, unfortunately, is not sufficient afaict: If a CPU gets brought up
> post-boot, the variable may need to be cleared again. Instead you
> ought to call mark_tsc_unstable().

Yeah, mark_tsc_unstable() is the right thing to do.

David
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