Re: regression introduced by - timers: fix itimer/many thread hang

[Frank Mayhar <fmayhar@google.com>]

On Mon, 2008-11-10 at 15:42 +0100, Peter Zijlstra wrote:
> On Mon, 2008-11-10 at 08:38 -0600, Christoph Lameter wrote:
> > Can we at least somehow make sure that nothing significantly happens in a
> > timer interrupt on a processor if the thread has not scheduled any events
> > or not odone any system calls?
> Do threads actually scale that far? I thought mmap_sem contention and
> other shared state would render threads basically useless on these very
> large machines.
> 
> But afaiu this stuff, the per-cpu loop is only done when an itimer is
> actually active.

Correct.

> The detail I've not looked at is, if when this itimer is indeed active
> and we are running 256 threads of the same application on all cpus do we
> then do the per-cpu loop for each tick on each cpu?

The answer to this question is, "that depends."  You can have an itimer
for a single thread or for the whole thread group.  In the former case,
it never happens; it only does the loops for the thread group case.  If
there is a thread group itimer then of course we have to sum the tick
count across all CPUs to determine whether the timer has expired.

Personally, I would argue that it's silly to have an itimer running when
you have many threads, and if you care about performance it's even
_more_ silly.  But it's sillier yet to be able to wedge the kernel by
running a program in user space.

As far as Christoph's concern regarding latency for 8- and 16-processor
systems, my belief (supported by data I can't discuss, sigh) is that the
loop adds negligible latency.  In fact, it can't really be discussed in
this way since the existing implementation adds *lots* of latency when
an itimer is running, since it sums the values across all threads.  I
never collected latency versus number of threads data but it's bad
enough that at about 4500 threads (on a dual amd64) it took longer than
a tick to do a tick's worth of processing.
-- 
Frank Mayhar <fmayhar@google.com>
Google, Inc.