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

[Lin Ming <ming.m.lin@intel.com>]

On Thu, 2010-05-13 at 16:38 +0800, Ingo Molnar wrote:
> * Lin Ming <ming.m.lin@intel.com> wrote:
> 
> > On Tue, 2010-05-11 at 17:18 +0800, Ingo Molnar wrote:
> > > * Peter Zijlstra <peterz@infradead.org> 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.
> > 
> > I still don't understand this sys_fd -> pmu lookup, would you please
> > explain it more detail?
> > 
> > struct pmu {
> > 	kobject kobj;
> > 	...
> > };
> > 
> > What I can imagine is,
> > 
> > 1. In userspace, sys_fd =
> > open("/sys/devices/system/cpu/event_source", ..), then sys_fd is passed
> > to sys_perf_event_open()
> 
> Yes, open() an event_source - or rather an event itself. For raw events there 
> has to be a separate event entry that can be opened.
> 
> I.e. we'd have a layout like:
> 
>  /sys/devices/system/cpu/events/cycles/id
>  /sys/devices/system/cpu/events/instructions/id
>  /sys/devices/system/cpu/events/raw/id
> 
> By making each event category a directory we gain the flexibility of 
> integrating tracepoints as well, for example:
> 
>  /sys/kernel/sched/events/wakeup/id
>  /sys/kernel/sched/events/wakeup/format
> 
> Where 'format' describes the event record layout:
> 
>  # cat /debug/tracing/events/sched/sched_wakeup/format 
>  name: sched_wakeup
>  ID: 59
>  format:
> 	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
> 	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
> 	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
> 	field:int common_pid;	offset:4;	size:4;	signed:1;
> 	field:int common_lock_depth;	offset:8;	size:4;	signed:1;
> 
> 	field:char comm[TASK_COMM_LEN];	offset:12;	size:16;	signed:1;
> 	field:pid_t pid;	offset:28;	size:4;	signed:1;
> 	field:int prio;	offset:32;	size:4;	signed:1;
> 	field:int success;	offset:36;	size:4;	signed:1;
> 	field:int target_cpu;	offset:40;	size:4;	signed:1;
> 
>  print fmt: "comm=%s pid=%d prio=%d success=%d target_cpu=%03d", REC->comm, REC->pid, REC->prio, REC->success, REC->target_cpu
> 
> > 2. In kernel, sys_file = <find the sys file structure with sys_fd>
> > 
> > 3. kobject = <retrieve the kobject from sys_file>
> > 
> > 4. pmu = container_of(kobject, struct pmu, kobj)
> > 
> > If my understanding is correct, then step 3 above seems strange. It's
> > not the typical usage of sys file.
> 
> I dont think it's stange - we demux from the generic sysfs object to the more 
> specific perf events related object. This is similar how driver specific sysfs 
> functionality does the demux as well.

OK, thanks for the explanation.