Re: [PATCH] Add some documentation on the perf sysfs ABI interface

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Sep 12, 2014 at 03:34:19PM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
> 
> Initial attempt of documenting the perf sysfs interface as
> an ABI. I also added some additional pointers hopefully useful
> to the users. Comments welcome.

My only worry is that its a little x86 centric and I'm not sure if that
is acceptable with the sysfs crowd, Greg?

Other than that it looks like a nice addition and I suppose other
popular archs can always add to it.

> Cc: Vince Weaver <vincent.weaver@maine.edu>
> Cc: jolsa@redhat.com
> v2: Various fixes. Fix cmask/inv (Stephane) Fixes from Randy Dunlap.
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> ---
>  Documentation/ABI/stable/sysfs-devices-perf | 98 +++++++++++++++++++++++++++++
>  1 file changed, 98 insertions(+)
>  create mode 100644 Documentation/ABI/stable/sysfs-devices-perf
> 
> diff --git a/Documentation/ABI/stable/sysfs-devices-perf b/Documentation/ABI/stable/sysfs-devices-perf
> new file mode 100644
> index 0000000..3fd9bc6
> --- /dev/null
> +++ b/Documentation/ABI/stable/sysfs-devices-perf
> @@ -0,0 +1,98 @@
> +Perf events enumeration in sysfs
> +
> +The perf events subsystem exports the format of hardware performance
> +counter events supported by perf events. The events can be accessed
> +using the perf_event_open() syscall. Each perf directory in devices
> +represents a distinct PMU (Performance Monitoring Unit), but not all
> +directories in this file are perf directories.
> +
> +What: /sys/devices/*/format/*
> +Description:
> +
> +Each file in format describes how to fill in an event attribute on the
> +current CPU for the perf_event_open syscall.  Multiple event
> +attributes may be overlapping and only be valid for some combination
> +of attributes (for example only for some event/umask combinations).
> +Most attributes are optional.
> +
> +Each field may have the following contents:
> +
> +CONFIG:START-END      Field consists of bits START-END in the perf_event_attr
> +		      CONFIG field
> +CONFIG:BIT	      Field consists of a single bit with index BIT in
> +		      CONFIG field
> +
> +Valid CONFIG fields are config, config1, config2. These map to the respective
> +64bit words in struct perf_event_attr.
> +
> +Typical attributes on a x86 platform
> +
> +event		Set the 8 bit event code (required)
> +umask		Set the 8 bit umask. Event code and umask together select a
> +		hardware event.
> +cmask		Set the 8 bit counter Mask. Only increment counters when at
> +		least cmask events happen during the same cycle.
> +inv		(1bit flag) Invert the cmask condition. Only valid with
> +		cmask>0.
> +edge		(1bit flag) Only increment the event when the condition
> +		changes (starts happening)
> +any		(1bit flag) Count on both threads of a core
> +pc		(1bit flag) Toggle the PMi pins when the condition happens
> +
> +Attributes available on some x86 platforms:
> +
> +in_tx		(1bit flag) Only count in a hardware transaction.
> +in_tx_cp	(1bit flag) Undo counts inside transaction when the
> +		transaction aborts.
> +ldlat		Set the load-use latency threshold for sampling loads.
> +		Note this is a load-use latency so includes pipeline delays.
> +offcore_rsp     Set an extra mask qualifying the type of offcore access.
> +		Only with OFFCORE_RESPONSE events. The actual mask is CPU model
> +		specific.
> +
> +For more details on the x86 attributes on Intel platforms please see
> +http://www.intel.com/sdm Volume 3, Chapter 18 and 19.  For more
> +details on the perf_event_attr struct please see the perf_event_open
> +manpage and include/uapi/linux/perf_event.h.
> +
> +What: /sys/devices/*/events/*
> +Description:
> +
> +Describe predefined events available in the CPU. Each file describes an event.
> +The format is attr=0xHEXNUM{,attr=0xHEXNUM}. Each attr is described in a config
> +file. Together all the attributes can be used to set up a valid event for the
> +perf_event_open syscall.
> +
> +Typically only a small subset of the CPU events is described in sysfs.
> +Some more events are available through predefined classes in perf_event_attr.
> +Even more events require filling in CPU specific values. The libraries referenced
> +below provide larger event lists.
> +
> +What: /sys/devices/*/type
> +Description:
> +
> +Decimal number: The PMU type to fill into perf_event_attr in the
> +type field to select the correct PMU.
> +
> +What: /sys/devices/*/perf_event_mux_interval_ms
> +Description:
> +
> +Decimal number: Set the counter multiplexing interval in ms. When more
> +events are active than the hardware directly supports perf events
> +multiplexes the event. By default (value 0) this is done on timer interrupts
> +(depending on the CONFIG_HZ setting) and not done while idle. This
> +allows to set a different frequency. Note that setting this to non 0
> +may impact idle time, as the event switches will wake up the CPUs now.
> +
> +What: /sys/devices/*/rdpmc
> +Description:
> +
> +[x86] Decimal number: When 1, allow the RDPMC instruction in user space
> +to read performance events that have been set up with perf. When 0
> +ring 3 RDPMC access is disallowed.
> +
> +Users: perf (tools/perf/*)
> +The following libraries provide more user friendly interfaces:
> +       PAPI (http://icl.cs.utk.edu/papi/)
> +       libpfm4 (http://perfmon2.sourceforge.net/)
> +       jevents (http://github.com/andikleen/pmu-tools)
> -- 
> 1.9.3
>