Re: [RFC][PATCH 3/9] perf: export registerred pmus via sysfs

[Lin Ming]

On Tue, 2010-05-11 at 17:31 +0800, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > On Tue, 2010-05-11 at 11:18 +0200, Ingo Molnar wrote:
> > > * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > 
> > > > The new PMUs will use a dynamic range that starts at PERF_TYPE_MAX.
> > > 
> > > I dont think we should use a dynamic range of event sources - it's a 
> > > completely useless indirection that has no meaning to humans.
> > > 
> > > As far as machine interfaces go a much cleaner approach would be to allow an 
> > > open fd to a sysfs file to be passed to sys_perf_event_open() - this would 
> > > identify the event source. This needs a small extension of the ABI but we 
> > > could thus get rid of the 'type' enumeration altogether and express _all_ 
> > > event sources via fds to sysfs files.
> > 
> > Whatever, that's almost identical. [...]
> 
> It's not identical: as we dont expose our mapping structure externally and 
> dont have to have some dynamic type ID allocation layer/mechanism. Also, using 
> fds is an elegant, Linuxish way of expressing some object's identity and 
> passing it along.

Don't understand this well enough.

Does it need to attach pmu info to the sys file?

For example, 

Does "perf top -e cycles" work as below?

1. get the pmu file of cycles events,
ie, /sys/devices/system/cpu/cpu_hw_events/event_source_id

2. fd = open("/sys/devices/system/cpu/cpu_hw_events/event_source_id")

3. perf_event_attr::type = fd | (1<<31)

4. In kernel, perf_event_lookup_pmu() finds pmu by fd -> sys file -> pmu


> 
> ( It also removes the possibility to intentionally or accidentally have type 
>   IDs that are not reachable via the sysfs and vica verse. )
> 
> > [...] What we can do is reserve perf_event_attr::type with bit 31 set for 
> > fd's and use the fd->file lookup instead of the type->pmu lookup.
> 
> Yeah, that sounds good.
> 
> 	Ingo

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