Re: [RFC][PATCH v2 06/11] perf: core, export pmus via sysfs

[Greg KH <greg@kroah.com>]

On Thu, May 20, 2010 at 09:52:32PM +0200, Peter Zijlstra wrote:
> On Thu, 2010-05-20 at 11:42 -0700, Greg KH wrote:
> > On Wed, May 19, 2010 at 09:14:36AM +0200, Peter Zijlstra wrote:
> 
> > > Since the CPU and memory controllers are (assumed) symmetric on the
> > > system, we get to add things like:
> > > 
> > > 
> > >   /sys/devices/system/cpu/cpu_event_source/
> > 
> > Wouldn't that really be:
> > 	/sys/devices/system/cpu/cpu0/cpu_event_source/
> > ?
> > 
> > /sys/devices/system/cpu is a "type" of devices in the system here, and
> > isn't an event source specific to the device itself?
> > 
> > Or is it for all cpus together?
> 
> All CPUs are assumed identical, and the perf syscall has task/cpu
> monitor targets. If the CPUs would not be identical (like Paul Mundt
> said SH might do) then it would make sense to have different
> event_sources for each cpu.

Ah, ok.

> > >  fd = open("/sys/devices/system/cpu/cpu_event_source/instructions/config");
> > >  attr->type = fd | PERF_TYPE_FD;
> > >  event_fd = perf_event_open(attr, ... );
> > >  close(fd);
> > > 
> > > From that one fd we can find to which 'event_source' it belongs and what
> > > particular config we need to use.
> > 
> > Ah, pass the fd of a sysfs file to sysfs to get the kobject.  Ick,
> > that's just, well, something that I never even considered someone would
> > need/want to do...
> 
> No, we don't pass the fd to sysfs, we pass the fd into a syscall.

Sorry, yes, that is what I was trying to say.  You then take that fd,
pass it to the sysfs core within the kernel, and get a kobject back.

> > sysfs exports single values just fine.  If you are starting to do more
> > complex things, like you currently are, maybe you shouldn't be in
> > sysfs...
> 
> Well, like said, I'm fine with it actually being single values, its just
> that Ingo suggested skipping a few syscalls.
> 
> Robert just suggested we could use the sysfs files as device nodes and
> have then open() return a perf_event fd. Its just that that would
> require we add a ioctl to change the perf_event_attr structure and
> attach it to a context.

Nope, sorry, that's not going to happen, that is not what sysfs is for.
No ioctls or device nodes in there please.

> > I can always knock up a eventfs for you do mount at /sys/kernel/events/
> > or something if you want :)
> 
> But that won't get us the nice device linkage, right?

True, you would loose that.

thanks,

greg k-h