Re: conflict between tickless and perfmon2

[Stephane Eranian <eranian@hpl.hp.com>]

Hello,

On Fri, Nov 09, 2007 at 07:40:31PM +0100, Thomas Gleixner wrote:
> On Fri, 9 Nov 2007, Peter Zijlstra wrote:
> 
> > On Fri, 2007-11-09 at 02:44 -0800, Stephane Eranian wrote:
> > > Hello,
> > > 
> > > We have identified a conflict between TICKLESS (CONFIG_NO_HZ) and
> > > the current perfmon2 implementation. The problem impacts system-wide
> > > sessions using timeout-based event set multiplexing.
> > > 
> > > Event set multiplexing allows monitoring tools to measure more events
> > > than there are actual performance counters on the processor. Events
> > > are grouped in sets which are then multiplexed onto the actual counters.
> > > Switching can be triggered either by a timeout or by a counter overflow.
> > > This is supported for per-thread and system-wide sessions.
> > > 
> > > For timeout-based switching, the duration expressed in nanoseconds is
> > > meant to represent wall-clock time in system-wide mode, and execution
> > > time in per-thread mode. Granularity is limited by HZ.
> > > 
> 
> Using a hrtimer is perfrectly fine, I'd say it's preferred over hooks in 
> some code which has absoluty no guarantee of being executed periodically 
> or even executed at all. OTOH it seems rather stupid to measure stuff 
> while the system is idle and doing nothing.
> 

I have now converted the timeout-based set mtuliplexing to use hrtimer instead.
The patch is available from the perfmon2 GIT tree on kernel.org.

with this patch, multiplexing works with tickless kernels for system-wide
sessions. All the arch specific hooks are gone.

For system-wide, the timeout is measurement wall-clock time. For per-thread,
it is measuring virtual time. I could not find a way to count virtual time
with hrtimer. Thus I ended up using a hrtimer/cpu and cancel/restore timeout
on context switch. I suspect there may be a better way of doing this but for
now it seems to work.

With this patch, timeout-bsed multiplexing should work on all arch. I have
test on i386. x86-64, ia64. Please try the other ones as well.

Couple of interfaces changes related to this patch:

  - switch timeout is only running between pfm_start/pfm_stop calls and when the
    context is not masked due to sampling overflows. It used to be running between
    pfm_load_contex/pfm_unload_context. This means that on architectures which allow
    start/stop for user level (e.g., IA-64), it is now necessary to call pfm_start and
    pfm_stop when using multiple sets. It is not really practical to combine set
    switching in the kernel with user level direct reading of the registers.

  - pfm_create_evtsets() fails if the timeout is not a multiple of the clock resolution.
    Using clock_getres(CLOCK_MONOTONIC) users can figure out the granularity and adjust
    the timeout accordingly.


-- 
-Stephane
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