Messages in this thread | | | Date | Fri, 23 Nov 2007 06:28:31 -0800 | From | Stephane Eranian <> | Subject | Re: conflict between tickless and perfmon2 |
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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.
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