Re: Measuring tools - top and interrupts

["Björn Steinbrink" <B.Steinbrink@gmx.de>]

On 2006.06.24 18:23:12 +0200, Mike Galbraith wrote:
> On Sat, 2006-06-24 at 17:40 +0200, Björn Steinbrink wrote:
> > On 2006.06.24 13:41:57 +0200, Mike Galbraith wrote:
> > > On Sat, 2006-06-24 at 11:52 +0200, Mike Galbraith wrote:
> > > > On Sat, 2006-06-24 at 11:21 +0200, Björn Steinbrink wrote:
> > > > > 
> > > > > The non-SMP call to update_process_times() is in do_timer_interrupt_hook(),
> > > > > so I guess the above is not the Right Thing to do.
> > > > 
> > > > Ah, there it is.  That's what I was looking for.  I figured that doing
> > > > what I did had to be wrong, but tried it for grins anyway... was pretty
> > > > surprised when it worked (kinda).
> > > 
> > > Calling update_process_times() in do_timer_interrupt_hook() flat does
> > > not work here.  Calling it in smp_local_timer_interrupt() works fine.  
> > > 
> > > Oh joy.
> > 
> > I can reproduce it now, seems to require CONFIG_4KSTACKS to fail. Can
> > you confirm that?
> 
> What a coincidence.  After trying a different compiler, and slogging
> through a bunch of assembler trying to figure out how the heck this can
> happen, I was just booting an 8k stack kernel (as a wild-ass guess;).
> 
> let's see.  Yeah, confirmed.

OK, it also depends on IO APIC being enabled and active, ie. noapic on
the kernel command line will fix it as well as disabling
CONFIG_X86_IO_APIC. That doesn't help me at all to understand why it
happens though.
The only difference with IO APIC disabled seems to be that the irq
doesn't get ACKed before update_process_times() gets called.
And your "fix" makes it being called outside of the xtime_lock spinlock
and with a slightly different stack usage AFAICT.
But none of these should make a difference, right?

Björn