> > > unfortunately, most of the time is being spent with clock interrupts blocked,
> > 
> > this reminds me PR/26108.
> > can you try the patch in the PR?
> 
> even without that patch, I don't see much difference in the clock drift
> when a 2 CPU machine is idle vs. when I have both CPUs in the kernel
> (and thus one would be waiting for the big lock).  to generate the latter
> load, I ran 5 copies of "dd if=/dev/zero of=/dev/null bs=100m count=10000"
> simultaneously.
> 
> so I'm not sure how to reproduce the problem.

i forgot how to reproduce.
(apparently i should have noted it in the PR :-)

i think it was something like the following, which involves
frequent context switches, thus biglock contentions in mi_switch.
dd if=/dev/zero bs=1|dd bs=1|dd bs=1| ... |dd bs=1 of=/dev/null

YAMAMOTO Takashi