Re: conflict between tickless and perfmon2

[Peter Zijlstra <peterz@infradead.org>]

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.
> 
> The current implementation for timeout is a simple hook on the timer
> interrupt path in apic_*.c:smp_local_timer_interrupt(). Unfortunately,
> this does not work when tickless is enabled: we get much less set
> switches than expected on an idle system.
> 
> It looks like a solution would be to change the implementation of
> timeout-based switching to use HR timers instead. Similar to what is
> done for ITIMER_REAL and ITIMER_VIRTUAL.
> 
> Unless someone has a better proposal, I will experiment with this on
> 2.6.24-rc2.

Might help if you CC the tickless folks :-)