[Xen-devel] xen_sched_clock problem in RHEL6b2, Was: [PATCH] x86: unconditionally mark TSC unstable under Xen

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

Hi Michael --

Not sure if you are the right person for this, but FYI
it appears that this bug is also in RHEL6 beta 2 and
might account for some weird issues I've seen running
RHEL6 betas as Xen guests.

Problem description:
http://lists.xensource.com/archives/html/xen-devel/2010-07/msg00241.html 
Jeremy's solution (forwarded below):
http://lists.xensource.com/archives/html/xen-devel/2010-07/msg00749.html 

Dan

> -----Original Message-----
> From: Jeremy Fitzhardinge [mailto:jeremy@xxxxxxxx]
> Sent: Wednesday, July 14, 2010 3:36 PM
> To: Jed Smith
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Jan Beulich
> Subject: [Xen-devel] Re: [PATCH] x86: unconditionally mark TSC unstable
> under Xen
> 
> On 07/14/2010 12:24 PM, Jed Smith wrote:
> > Jeremy, Jan - what do you think?  Is this a bad move?  I feel like
> there
> > is a consequence to this that I am unaware of, but it fixes my issue.
> >
> 
> Ah, well that's interesting.
> 
> There's a couple of comments:
> 
>    1. you can't do this with just a compile-time test, since the same
>       kernel can also boot native
>    2. nothing in a Xen PV domU environment should be using the tsc
>       directly, so this shouldn't have an effect.  If something is
> using
>       the tsc we should track it down.
> 
> I wonder, however, if you're getting the same result as Jan's
> suggestion
> of making sched_clock unstable by making the tsc unstable.
> 
> In that case, this patch may help:
> 
> Subject: [PATCH] xen: disable xen_sched_clock by default
> 
> xen_sched_clock only counts unstolen time.  In principle this should
> be useful to the Linux scheduler so that it knows how much time a
> process
> actually consumed.  But in practice this doesn't work very well as the
> scheduler expects the sched_clock time to be synchronized between
> cpus.  It also uses sched_clock to measure the time a task spends
> sleeping, in which case "unstolen time" isn't meaningful.
> 
> So just use plain xen_clocksource_read to return wallclock nanoseconds
> for sched_clock.
> 
> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
> 
> diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig
> index b83e119..6a09f01 100644
> --- a/arch/x86/xen/Kconfig
> +++ b/arch/x86/xen/Kconfig
> @@ -29,6 +29,10 @@ config XEN_SAVE_RESTORE
>         depends on XEN && PM
>         default y
> 
> +config XEN_SCHED_CLOCK
> +       bool
> +       default n
> +
>  config XEN_DEBUG_FS
>       bool "Enable Xen debug and tuning parameters in debugfs"
>       depends on XEN && DEBUG_FS
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index 4b57c0b..242a230 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -939,7 +939,11 @@ static const struct pv_time_ops xen_time_ops
> __initdata = {
>       .set_wallclock = xen_set_wallclock,
>       .get_wallclock = xen_get_wallclock,
>       .get_tsc_khz = xen_tsc_khz,
> +#ifdef CONFIG_XEN_SCHED_CLOCK
>       .sched_clock = xen_sched_clock,
> +#else
> +     .sched_clock = xen_clocksource_read,
> +#endif
>  };
> 
>  static const struct pv_cpu_ops xen_cpu_ops __initdata = {
> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> index 9d1f853..32dc3dc 100644
> --- a/arch/x86/xen/time.c
> +++ b/arch/x86/xen/time.c
> @@ -154,6 +154,7 @@ static void do_stolen_accounting(void)
>       account_idle_ticks(ticks);
>  }
> 
> +#ifdef CONFIG_XEN_SCHED_CLOCK
>  /*
>   * Xen sched_clock implementation.  Returns the number of unstolen
>   * nanoseconds, which is nanoseconds the VCPU spent in RUNNING+BLOCKED
> @@ -191,7 +192,7 @@ unsigned long long xen_sched_clock(void)
> 
>       return ret;
>  }
> -
> +#endif
> 
>  /* Get the TSC speed from Xen */
>  unsigned long xen_tsc_khz(void)
> 
> Thanks,
>       J
> 
> 
> 
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