Re: [patch] prefer TSC over PM Timer

From: dean gaudet
Date: Mon Nov 15 2004 - 22:22:36 EST

On Mon, 15 Nov 2004, john stultz wrote:

> I understand your frustration.
> While there are a great number of systems that can use the TSC, cpufreq
> scaling laptops, and a number of SMP and NUMA systems cannot use it as a
> time source. Additinoally its difficult to detect when its wrong as
> there are a reasonable number of systems that frequently miss timer
> ticks. Although it is much slower, ACPI PM is just more reliable across
> the broad spectrum of systems.

i'm having a difficult time getting a centrino w/cpufreq to do anything
bad with tsc while i'm imposing loads which cause the frequency to flutter
around (i've got ondemand governor going). maybe i need to do something
more detailed like have ntp running against a solid time source while i do
all these and let it run for longer to look for drift. suggestions

> With your patch, ACPI PM would never be selected (as TSC always wins
> when available, and it will be available on all ACPI enabled i386
> systems). So its just the same as disabling CONFIG_X86_PM_TIMER, so why
> not just do that?

my patch lets you use "clock=pmtmr" if you want it.

> Do note, using the "clock=tsc" boot option, you can easily force the
> system to use the TSC.

right -- except i think the default is the opposite of what it should be
for a generic kernel. i think more systems are served better by using tsc
than those that need clock=pm... NUMA systems are rare (with custom
kernels/etc), and if my experience with the centrino is valid then newer
laptops aren't having this tsc/cpufreq problem.

> > note: when timer_tsc discovers inaccuracy after boot it falls back to
> > timer_pit ... timer_pit is twice as expensive as timer_pm, and it'd be
> > cool if timer_tsc could fall back to timer_pm... but by that point in time
> > all the __init stuff is gone, so i can't see how to init timer_pm. this
> > would be a more ideal solution.
> Well, the lost-ticks/pit fallback code isn't all that robust. We have
> two unreliable time sources where we try to sort out which one is wrong
> by using the other. I worry we'd have to implement something like NTP in
> the kernel in order to correctly choose the best working time source.

yeah that does sound unfortunate... it's almost like we should initialize
timer_pm whenever it is there so it can be used for these calibration

> I would however, support a patch that selected the TSC over the ACPI PM
> time source when CONFIG_CPUFREQ and CONFIG_SMP were N. That's fairly
> safe.

i'm looking for a solution that generic distribution kernels can use...

honestly my selfish motivation is to get efficeon/crusoe treated properly
-- they support a fixed TSC rate which does not vary with frequency (which
many people fault us for, but the reality is that fixed TSC is the only
viable solution for a processor which can vary power consumption without
the involvement of the kernel). i'd advocate a patch like the one
below... but it feels wrong.

i suppose one way to solve all this is to punt the whole thing to userland
and let someone write a tool which either uses a database or runs code
to figure out which timer is best and sticks that into grub/lilo/whatever.


Signed-off-by: dean gaudet <dean@xxxxxxxxxx>

--- linux-2.6.10-rc2/arch/i386/kernel/timers/timer_pm.c.orig 2004-11-15 23:28:30.000000000 -0800
+++ linux-2.6.10-rc2/arch/i386/kernel/timers/timer_pm.c 2004-11-16 03:05:52.000000000 -0800
@@ -107,6 +107,13 @@
if (!cpu_has_tsc)
return -ENODEV;

+ /*
+ * Transmeta CPUs have a fixed rate TSC, so prefer tsc
+ * unless the user specifically requests pmtmr.
+ */
+ if (!override[0] && boot_cpu_data.x86_vendor == X86_VENDOR_TRANSMETA)
+ return -ENODEV;
/* "verify" this timing source */
value1 = read_pmtmr();
for (i = 0; i < 10000; i++) {
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