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

[Ingo Molnar <mingo@elte.hu>]

* Greg KH <greg@kroah.com> wrote:

> [...]
>
> I can always knock up a eventfs for you do mount at /sys/kernel/events/ or 
> something if you want :)

eventfs was my first idea, until Peter convinced me that we want sysfs :-)

One important aspect would be to move it into the physical topology. Graphics 
card? It might have events. PCI device? It might have events. Southbridge? It 
might have a PMU and events. CPU? It has a PMU.

Especially when it comes to complex physical topologies on larger systems, we 
eventually want to visualize things in tooling as well - as a tree of the 
physical topology. Also, physical topologies will only become more complex, so 
we dont want to detach events from them.

> 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...

This is really like a read-only attributes, and it would be multi-line only 
for the event format descriptor - a genuinely new aspect: a flexible ABI 
descriptor.

It's an attribute for a very good purpose: flexible ABI with a user-space that 
interprets new format descriptions automatically. This is not just theory, for 
example perf trace does this today, and you can write scripts with old tools 
for a new event that shows up in a new kernel, without rebuilding the tools.

Here is an example of a format descriptor:

# 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

Also, we already have quite a few multi-line files in sysfs, for example:

$ cat /sys/devices/pnp0/00:09/options
Dependent: 00 - Priority preferred
  port 0x378-0x378, align 0x0, size 0x8, 16-bit address decoding
  port 0x778-0x778, align 0x0, size 0x8, 16-bit address decoding
  irq 7 High-Edge
  dma 3 8-bit compatible
Dependent: 01 - Priority acceptable
  port 0x378-0x378, align 0x0, size 0x8, 16-bit address decoding
  port 0x778-0x778, align 0x0, size 0x8, 16-bit address decoding
  irq 3,4,5,6,7,10,11,12 High-Edge
  dma 0,1,2,3 8-bit compatible
Dependent: 02 - Priority acceptable
  port 0x278-0x278, align 0x0, size 0x8, 16-bit address decoding
  port 0x678-0x678, align 0x0, size 0x8, 16-bit address decoding
  irq 3,4,5,6,7,10,11,12 High-Edge
  dma 0,1,2,3 8-bit compatible
Dependent: 03 - Priority acceptable
  port 0x3bc-0x3bc, align 0x0, size 0x4, 16-bit address decoding
  port 0x7bc-0x7bc, align 0x0, size 0x4, 16-bit address decoding
  irq 3,4,5,6,7,10,11,12 High-Edge
  dma 0,1,2,3 8-bit compatible

$ cat /sys/devices/pci0000:00/0000:00:1a.7/pools
poolinfo - 0.1
ehci_sitd           0    0   96  0
ehci_itd            0    0  160  0
ehci_qh             4   42   96  1
ehci_qtd            4   42   96  1
buffer-2048         0    0 2048  0
buffer-512          0    0  512  0
buffer-128          0    0  128  0
buffer-32           1  128   32  1

In fact uevents have multi-line attributes as well:

$ cat /sys/devices/pci0000:00/0000:00:1a.1/usb4/uevent
MAJOR=189
MINOR=384
DEVNAME=bus/usb/004/001
DEVTYPE=usb_device
DRIVER=usb
DEVICE=/proc/bus/usb/004/001
PRODUCT=1d6b/1/206
TYPE=9/0/0
BUSNUM=004
DEVNUM=001

Thanks,

	Ingo