[Xen-devel] RE: [PATCH] clocksource=tsc

["Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>]

I meant to add... although this appears to be new functionality
and thus too late for 3.3, I'd like to argue that it is really
a bug fix:  Let's say there are good-tsc machines and bad-tsc
machines.  Prior to cset 17716 ("hvm: Build guest timers on
monotonic system time") all guest platform timer reads were
built directly on top of physical TSC.  This was a big problem
for bad-tsc SMP machines, but fine for good-tsc machines.  Cset 17716
made things better for bad-tsc machines, but actually made them
worse for good-tsc SMP machines (because of the inter-processor
skew).

So users with good-tsc machines moving from a pre-3.3 release
to 3.3 may see time "stopped" in 3.3 when there was no problem
with pre-3.3.    The clocksource=tsc patch partially
fixes this "regression" for good-tsc machines... at least it
provides a boot option to fix it.

Thanks,
Dan

> -----Original Message-----
> From: Dan Magenheimer [mailto:dan.magenheimer@xxxxxxxxxx]
> Sent: Saturday, July 12, 2008 3:38 PM
> To: Xen-Devel (E-mail); Keir Fraser; dan.magenheimer@xxxxxxxxxx
> Cc: Dave Winchell
> Subject: [PATCH] clocksource=tsc
> 
> 
> Attached patch adds clocksource=tsc boot option that
> uses TSC as clocksource.  This option should only be
> used on machines where TSC is known to be synchronized
> across all processors.  A future TODO is to dynamically
> determine if this is the case.
> 
> TSC may "beat" with another clocksource, resulting in
> cross-processor Xen system time skew.  This skew can
> be visible in PV guests and can appear as "time is stopped"
> in hvm guests.  On some systems, this patch can reduce skew
> by 30x or more.
> 
> Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
> 
> ===================================
> Thanks... for the memory
> I really could use more / My throughput's on the floor
> The balloon is flat / My swap disk's fat / I've OOM's in store
> Overcommitted so much
> (with apologies to the late great Bob Hope)
>

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