Messages in this thread | | | Subject | Re: [PATCH/RFC 1/2] perfcounters: provide a way to read the current value of interrupting counters | From | Peter Zijlstra <> | Date | Tue, 17 Mar 2009 08:32:49 +0100 |
| |
On Tue, 2009-03-17 at 16:42 +1100, Paul Mackerras wrote: > Impact: new feature > > At present, if the user specifies hw_event->record_type == > PERF_RECORD_IRQ or PERF_RECORD_GROUP when creating a counter, reads from > the counter will return records from the interrupt event queue for the > counter. This means that there is no way to find out the current value > of the counter. Also, using the record_type is slightly problematic in > that what actually determines whether the counter generates interrupts > is whether hw_event->irq_period is non-zero or not.
I've never found that to be a problem, I've always read PERF_RECORD_IRQ as PERF_RECORD_SINGLE or somesuch in that it will give a single counter output, as opposed to PERF_RECORD_GROUP which will give a tuple.
> This provides a way for users to get a second fd for an interrupting > counter, which has a different set of file operations, set up so that > reads on the second (or "clone") fd return the counter value rather than > reading the interrupt event queue. The way to get the clone fd is like > this: > > clone_fd = sys_perf_counter_open(NULL, 0, 0, counter_fd, 0);
I'm not sure I understand why. It seems to me you're either interested in sample data, that is {tid,ip,counter} like things, or you want raw count values.
These two cases seem clearly distinct and provided for. Why are you mixing them?
> This will enable us to use a wider range of values in record_type in > future, allowing the user to specify what information they want recorded > on an interrupt.
This seems unrelated, what will stop us now from adding record_type values? PERF_RECORD_CALLCHAIN is one in particular I'd like to add soon.
> Internally, we now potentially have multiple struct files pointing to > the one struct counter, which could lead to lifetime issues. We avoid > any such issues by having the clone files keep a reference to the > original file. The reference is dropped when the clone file is closed. > Thus the original file can never be released while there is any clone > file still open.
This is a bit bothersome, as we then have no unique identifier anymore.
Currently the group record type writes things like {hw_event->type, counter} which is ambiguous since we really have a 65bit id space. So I was thinking of making that {fd, counter} to at least have a unique identifier in there.
| |