Re: [rfc] Describe events in a structured way via sysfs

[Corey Ashford <cjashfor@linux.vnet.ibm.com>]

On 07/19/2010 10:48 PM, Lin Ming wrote:
> On Sat, 2010-07-17 at 08:20 +0800, Corey Ashford wrote:
>> On 07/02/2010 01:06 AM, Lin Ming wrote:
>>> On Tue, 2010-06-29 at 18:26 +0800, Ingo Molnar wrote:
>>>> * Lin Ming<ming.m.lin@intel.com>   wrote:
>>>>
>>>>>> Also, we can (optionally) consider 'generic', subsystem level events to
>>>>>> also show up under:
>>>>>>
>>>>>>      /sys/bus/pci/drivers/i915/events/
>>>>>>
>>>>>> This would give a model to non-device-specific events to be listed one
>>>>>> level higher in the sysfs hierarchy.
>>>>>>
>>>>>> This too would be done in the driver, not by generic code. It's generally
>>>>>> the driver which knows how the events should be categorized.
>>>>>
>>>>> This is a bit difficult. I'd like not to touch TRACE_EVENT(). [...]
>>>>
>>>> We can certainly start with the simpler variant - it's also the more common
>>>> case.
>>>>
>>>>> [...] How does the driver know if an event is 'generic' if TRACE_EVENT is
>>>>> not touched?
>>>>
>>>> Well, it's per driver code which creates the 'events' directory anyway, so
>>>> that code decides where to link things. It can link it to the per driver kobj
>>>> - or to the per subsys kobj.
>>>>
>>>>>> I'd imagine something similar for wireless drivers as well - most
>>>>>> currently defined events would show up on a per device basis there.
>>>>>>
>>>>>> Can you see practical problems with this scheme?
>>>>>
>>>>> Not now. I may find some problems when write more detail code.
>>>>
>>>> Ok. Feel free to post RFC patches (even if they are not fully complete yet),
>>>> so that we can see how things are progressing.
>>>>
>>>> I suspect the best approach would be to try to figure out the right sysfs
>>>> placement for one or two existing driver tracepoints, so that we can see it
>>>> all in practice. (Obviously any changes to drivers will have to go via the
>>>> relevant driver maintainer tree(s).)
>>>
>>> Well, take i915 tracepoints as an example, the sys structures as below
>>>
>>> /sys/class/drm/card0/events/
>>> |-- i915_gem_object_bind
>>> |   |-- enable
>>> |   |-- filter
>>> |   |-- format
>>> |   `-- id
>> ...
>>
>> Hi Lin,
>>
>> Sorry for my late reply on this thread.  I had missed these posts
>> earlier because I had an email filter that was set to look for messages
>> with "perf" in the subject, and so I missed this entire thread.
>
> Sorry for my late reply too.
> I have been busy with some other stuff. Hope I can send a more
> functional patches this week.
>
>>
>> With your example here, let's say I want to open this event with the
>> perf_events ABI... how would I go about doing that?  Have you figured
>> out whether the caller would read the id and pass that into the
>> interface, or perhaps pass in the fd of the id file (or perhaps the fd
>> of the specific event directory).
>
> Please just ignore my above example. Now I have some uncompleted new
> patches to export hardware/software/tracepoint events via sysfs, like
> below.
>
> The event path is passed in with perf's "-e" option, for example
> perf record -e /sys/kernel/events/page-faults --<some commands>
>
> The caller reads config and type and pass them into perf_event_attr.
>
> 1. Hardware events
> /sys/devices/system/cpu/cpu0...cpuN/events
> |-- L1-dcache-load-misses  ===>  event name
> |   |-- config             ===>  config value for the event
> |   `-- type               ===>  event type
> |-- cycles
> |   |-- config
> |   `-- type
> .....
>
> 2. Software events
> /sys/kernel/events
> |-- page-faults
> |   |-- config
> |   `-- type
> |-- context-switches
> |   |-- config
> |   `-- type
> ....
>
> 3. Tracepoint events
> /sys/devices/pci0000:00/0000:00:02.0/events
> |-- i915_gem_object_create
> |   |-- config
> |   `-- type
> |-- i915_gem_object_bind
> |   |-- config
> |   `-- type
> ....
> ....
> /sys/devices/system/kvm/kvm0/events
> |-- kvm_entry
> |   |-- config
> |   `-- type
> |-- kvm_hypercall
> |   |-- config
> |   `-- type
> ....
> ....
>
>>
>> Also, I see the filter and format fields here.  Would the caller write
>> to these fields to set them up?  What's the format of the data that's
>> written to them?  Would it be totally device dependent?  It seems like
>> there should be a way for a user space tool to discover what can be
>> programmed into the filter and format fields.
>
> Now only read-only event attributes(config and type) are exported.
> I want to first make some minimal functional patches. Then to implement
> the complex writable attributes.

I'm not seeing the value of writable attributes in sysfs at this point. 
  Wouldn't that disconnect the event opening between the syscall and the 
writing of attributes in user space, with no real way to tie them 
together?  For example, what if two users wrote to the same attribute 
with different values... which one would take precedence when you go to 
do the open syscall?  I think all of the attribute data should be in the 
open call, and sysfs should be read-only.

Earlier, I briefly presented an idea that would allow a caller to read 
attribute formatting information, such as a shift and mask value, which 
would allow the caller to build up a more complex .config value, 
possibly extending into a new attr field - .config_extra[n] as dictated 
by the shift value; shift values greater than 63 would place the 
attribute into .config_extra[shift amount / 64] shifted by shift amount 
% 64.  It's not the prettiest interface, but I think it could work and 
would be extensible.

- Corey