Re: nosmp/maxcpus=0 or 1 -> TSC unstable

[Pete Wyckoff]

dean@xxxxxxxxxx wrote on Sat, 12 Jan 2008 11:48 -0800:
> if i boot an x86 64-bit 2.6.24-rc7 kernel with nosmp, maxcpus=0 or 1 it 
> still disables TSC :)
> 
> Marking TSC unstable due to TSCs unsynchronized
> 
> this is an opteron 2xx box which does have two cpus and no clock-divide in 
> halt or cpufreq enabled so TSC should be fine with only one cpu.
> 
> pretty sure this is the culprit is that num_possible_cpus() > 1, which 
> would mean cpu_possible_map contains the second cpu... but i'm not quite 
> sure what the right fix is... or perhaps this is all intended.

We've seen the same problem.  We use gettimeofday() for timing of
network-ish operations on the order of 10-50 us.  But not having
the TSC makes gettimeofday() itself very slow, on the order of 30 us.

Here's what we've been using for quite a few kernel versions.  I've
not tried to submit it for fear that it could break some other
scenario, as you suggest.  Although in hotplug scenarios, this
function unsynchronized_tsc() should get rerun and disable TSC if
more processors arrive.

At least count this as a "me too".

		-- Pete