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SubjectRe: [RFC][PATCH v2 06/11] perf: core, export pmus via sysfs
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On Wed, 2010-05-19 at 10:48 +0800, Greg KH wrote:
> On Wed, May 19, 2010 at 10:34:55AM +0800, Lin Ming wrote:
> > On Wed, 2010-05-19 at 04:05 +0800, Greg KH wrote:
> > > On Wed, May 19, 2010 at 01:46:42AM +0000, Lin Ming wrote:
> > > > Now only exports cpu hardware events.
> > > >
> > > > For each PMU, there are 2 sysfs dirs: event_source and events.
> > > >
> > > > For example,
> > > > /sys/devices/system/cpu/event_source/
> > > > /sys/devices/system/cpu/events/
> > > >
> > > > $ tree /sys/devices/system/cpu/event_source/
> > > > /sys/devices/system/cpu/event_source/
> > > > `-- id
> > > >
> > > > $ tree /sys/devices/system/cpu/events/
> > > > /sys/devices/system/cpu/events/
> > > > |-- L1-dcache-load-misses
> > > > | |-- event_source -> ../../event_source
> > >
> > > What is this symlink for? Is it needed as they all seem to point to the
> > > same thing.
> >
> > The symlink is used to find the "event_source" of the "event".
> >
> > /sys/devices/system/cpu/event_source/
> >
> > /sys/devices/system/cpu/events/
> > |-- L1-dcache-load-misses
> > | |-- event_source -> ../../event_source
> >
> > For above example, "event_source" is the cpu pmu and the "event" is
> > L1-dcache-load-misses.
> >
> > Yes, they point to the same thing, because all the events
> > under /sys/devices/system/cpu/events/* is monitored by the same event
> > source, ie, the cpu pmu(/sys/devices/system/cpu/event_source/).
>
> Then, why have a symlink if they are all going to be the same?

So how about move the symlink to parent directory, like

/sys/devices/system/cpu/event_source/

/sys/devices/system/cpu/events/
|-- event_source -> ../event_source

>
> > > I'm still not really understanding what exactly you are trying to show
> > > in the sysfs tree here anyway, perhaps the documentation update will
> > > clear that up for me.
> >
> > To support multiple pmus
>
> What is a "pmu"?

Performance Monitoring Unit

>
> > (or call them event source),
>
> What is an "event source"?

The source who generates the event.
The event includes hardware events, cache-misses, bus-cycles,
interrupts, error conditions, hotplug......
and software events, context-switches, page-faults, sched events......

>
> > I want to show all
> > the pmus and events in the sysfs tree.
>
> Why do these things need to be in sysfs? What benifit is it going to
> have?

Kernel to provide the information of the event source and
hardware/software events via sysfs and user-space can get
this info from sysfs.

>
> > For cpu pmu,
> > /sys/devices/system/cpu/event_source/*
> > /sys/devices/system/cpu/events/*
>
> What is a "cpu pmu"?

The performance monitoring unit in cpu.

>
> > For node pmu,
> > /sys/devices/system/node/event_source/*
> > /sys/devices/system/node/events/*
>
> What is a "node pmu"?

The pmu shared by logical cpus in a package, for example, Nehalem uncore
pmu, PowerPc nest units.

>
> > For mce,
> > /sys/kernel/events/mce/*
>
> What? No, don't create /sys/kernel/events/. Is that in this patchset?

No, not in this patchset.

> Ick, that's such a "general" name that it is going to get confusing very
> quickly. Heck, I still don't know what this "event" thing is yet :)
>
> > and so on...the exact placement is not yet final.
>
> Again, why do you need/want anything in sysfs in the first place?
> What problem is it going to solve? Who is going to benifit? Why do
> they care? What is this whole thing about?
>
> thanks,
>
> greg k-h



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