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

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

> I'm sympathetic to the patch, but it looks like an ugly hack 
> right now.

> Since it should clearly only affect behaviour when the new
> clocksource is explicitly selected...

Actually the ugliness is because the read_counter function
and plt_mask are both u32 and I worried that changing them to
a u64 would be too invasive.

So here's two possible replacement patches, one with those
switched to u64 and the other with separate 32- and 64-bit
read_counter functions.  Both are boot-tested with and
without clocksource=tsc.

Dan

> -----Original Message-----
> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Sent: Monday, July 14, 2008 3:24 AM
> To: dan.magenheimer@xxxxxxxxxx; Xen-Devel (E-mail)
> Cc: Dave Winchell
> Subject: Re: [PATCH] clocksource=tsc
> 
> 
> I'm sympathetic to the patch, but it looks like an ugly hack 
> right now. Most
> of the clocksource_is_tsc special cases will fall away if 
> you, e.g., have
> platform_timer_stamp = stime_platform_stamp = local_tsc_stamp =
> stime_local_stamp = 0; and provide a plt->read_counter() function; and
> accept that for now you will unnecessarily take the 
> platform-timer spinlock;
> etc. Also it is platform timer 5, not 4!
> 
> Clean it up and it'll go in. Since it should clearly only 
> affect behaviour
> when the new clocksource is explicitly selected, I'll take this patch
> further into 3.3 freeze than usual -- it's more important to 
> me that it's
> clean.
> 
>  Thanks,
>  Keir
> 
> On 12/7/08 22:38, "Dan Magenheimer" 
> <dan.magenheimer@xxxxxxxxxx> wrote:
> 
> > 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)
> 
> 
>

Attachment: tscstable2.patch
Description: Binary data

Attachment: tscstable3.patch
Description: Binary data

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