[rfc] Describe events in a structured way via sysfs

[Ingo Molnar <mingo@elte.hu>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, 2010-05-20 at 16:12 -0700, Greg KH wrote:

> > How deep in the device tree are you really going to be 
> > caring about?  It sounds like the large majority of 
> > events are only going to be coming from the "system" 
> > type objects (cpu, nodes, memory, etc.) and very few 
> > would be from things that we consider a 'struct 
> > device' today (like a pci, usb, scsi, or input, etc.)
> 
> The general noise I hear from the hardware people is 
> that we'll see more and more device-level stuff - bus 
> bridges/controller and actual devices (GPUs, NICs etc.) 
> will be wanting to export performance metrics.

There's (much) more:

 - laptops want to provide power level/usage metrics,

 - we could express a lot of special, lower level 
   (transport specific) disk IO stats via events as well - 
   without having to push those stats to a higher level 
   (where it might not make sense). Currently such kinds
   of stats/metrics are very device/subsystem specific 
   way, if they are provided at all.

Also, we already have quite a few per device tracepoints 
upstream. Here are a few examples:

 - GPU tracepoints (trace_i915_gem_request_submit(), etc.)
 - WIFI tracepoints (trace_iwlwifi_dev_ioread32(), etc.)
 - block tracepoints (trace_block_bio_complete())

So these would be attached to:

  # GEM events of drm/card0:
  /sys/devices/pci0000:00/0000:00:02.0/drm/card0/events/i915_gem_request_submit/

  # Wifi-ioread events of wlan0:
  /sys/devices/pci0000:00/0000:00:1c.1/0000:03:00.0/net/wlan0/events/iwlwifi_dev_ioread32/

  # whole sdb disk events:
  /sys/block/sdb/events/block_bio_complete/

  # sdb1 partition events:
  /sys/block/sdb/sdb1/events/block_bio_complete/

And we also have 'software nodes' in /sys that have events 
upstream here and today. For example for SLAB we already 
have kmalloc/kfree tracepoints (trace_kmalloc() and 
trace_kfree()):

  # all kmalloc events:
  /sys/kernel/slab/events/

  # kmalloc events for sighand_cache:
  /sys/kernel/slab/sighand_cache/events/kmalloc/

  # kfree events for sighand_cache:
  /sys/kernel/slab/sighand_cache/events/kfree/

In general the set of events we have upstream is growing 
along an exponential curve (there's over a hundred now, 
via tracepoints).

They are either logically attached to the hardware 
topology of the system (as in the first set of examples 
above), or ae attached to the software/subsystem object 
topology of the kernel (some examples of which are 
described in the second set of examples above).

Sometimes there are aliasing/filtering relationship 
between events, which is expressed very well via the 
hierarchy and granularity of /sysfs.

New events would go into that topology there in a natural 
way.

For example general hugepage tracepoints (should we 
introduce any) would go into the existing hugepage node:

  /sys/kernel/mm/hugepages/events/...

All in one, all these existing and future events, both of 
hardware and software type, are literally begging to be 
attached to nodes in /sys :-)

If we created a separate eventfs for it we'd have to start 
with duplicating all the topology/hiearchy/structure that 
is present in sysfs already. (and dilluting /sys's 
utility)

That would be a bad thing, so it would be nice if we found 
a workable solution here. We could split up the record 
format some more:

 /sys/kernel/sched/events/sched_wakeup/format/
 /sys/kernel/sched/events/sched_wakeup/format/common_type/
 /sys/kernel/sched/events/sched_wakeup/format/common_flags/
 /sys/kernel/sched/events/sched_wakeup/format/common_preempt_count/
 /sys/kernel/sched/events/sched_wakeup/format/common_pid/
 /sys/kernel/sched/events/sched_wakeup/format/common_lock_depth/
 /sys/kernel/sched/events/sched_wakeup/format/comm/
 /sys/kernel/sched/events/sched_wakeup/format/pid/
 /sys/kernel/sched/events/sched_wakeup/format/prio/
 /sys/kernel/sched/events/sched_wakeup/format/success/
 /sys/kernel/sched/events/sched_wakeup/format/target_cpu/

Into single-value files. But this would add significant 
parsing overhead (plus significant allocation overhead), 
for no tangible benefit.

The problem with /proc was always the lack of standard 
structure and the lack of performance - while the format 
file is about _more_ structure.

Increasing structure parsing overhead does not look like 
the right answer to that problem.

Hm?

	Ingo