Re: [PATCH v2 0/5] bpf: Introduce the new ability of eBPF programs to access hardware PMU counter

Daniel Borkmann
2015-07-23T23:34:07Z

On 07/22/2015 10:09 AM, Kaixu Xia wrote:
> Previous patch v1 url:
> https://lkml.org/lkml/2015/7/17/287

[ Sorry to chime in late, just noticed this series now as I wasn't in Cc for
the core BPF changes. More below ... ]

> This patchset allows user read PMU events in the following way:
> 1. Open the PMU using perf_event_open() (for each CPUs or for
> each processes he/she'd like to watch);
> 2. Create a BPF_MAP_TYPE_PERF_EVENT_ARRAY BPF map;
> 3. Insert FDs into the map with some key-value mapping scheme
> (i.e. cpuid -> event on that CPU);
> 4. Load and attach eBPF programs as usual;
> 5. In eBPF program, get the perf_event_map_fd and key (i.e.
> cpuid get from bpf_get_smp_processor_id()) then use
> bpf_perf_event_read() to read from it.
> 6. Do anything he/her want.
>
> changes in V2:
> - put atomic_long_inc_not_zero() between fdget() and fdput();
> - limit the event type to PERF_TYPE_RAW and PERF_TYPE_HARDWARE;
> - Only read the event counter on current CPU or on current
> process;
> - add new map type BPF_MAP_TYPE_PERF_EVENT_ARRAY to store the
> pointer to the struct perf_event;
> - according to the perf_event_map_fd and key, the function
> bpf_perf_event_read() can get the Hardware PMU counter value;
>
> Patch 5/5 is a simple example and shows how to use this new eBPF
> programs ability. The PMU counter data can be found in
> /sys/kernel/debug/tracing/trace(trace_pipe).(the cycles PMU
> value when 'kprobe/sys_write' sampling)
>
> $ cat /sys/kernel/debug/tracing/trace_pipe
> $ ./tracex6
> ...
> cat-677 [002] d..1 210.299270: : bpf count: CPU-2 5316659
> cat-677 [002] d..1 210.299316: : bpf count: CPU-2 5378639
> cat-677 [002] d..1 210.299362: : bpf count: CPU-2 5440654
> cat-677 [002] d..1 210.299408: : bpf count: CPU-2 5503211
> cat-677 [002] d..1 210.299454: : bpf count: CPU-2 5565438
> cat-677 [002] d..1 210.299500: : bpf count: CPU-2 5627433
> cat-677 [002] d..1 210.299547: : bpf count: CPU-2 5690033
> cat-677 [002] d..1 210.299593: : bpf count: CPU-2 5752184
> cat-677 [002] d..1 210.299639: : bpf count: CPU-2 5814543
> <...>-548 [009] d..1 210.299667: : bpf count: CPU-9 605418074
> <...>-548 [009] d..1 210.299692: : bpf count: CPU-9 605452692
> cat-677 [002] d..1 210.299700: : bpf count: CPU-2 5896319
> <...>-548 [009] d..1 210.299710: : bpf count: CPU-9 605477824
> <...>-548 [009] d..1 210.299728: : bpf count: CPU-9 605501726
> <...>-548 [009] d..1 210.299745: : bpf count: CPU-9 605525279
> <...>-548 [009] d..1 210.299762: : bpf count: CPU-9 605547817
> <...>-548 [009] d..1 210.299778: : bpf count: CPU-9 605570433
> <...>-548 [009] d..1 210.299795: : bpf count: CPU-9 605592743
> ...
>
> The detail of patches is as follow:
>
> Patch 1/5 introduces a new bpf map type. This map only stores the
> pointer to struct perf_event;
>
> Patch 2/5 introduces a map_traverse_elem() function for further use;
>
> Patch 3/5 convets event file descriptors into perf_event structure when
> add new element to the map;

So far all the map backends are of generic nature, knowing absolutely nothing
about a particular consumer/subsystem of eBPF (tc, socket filters, etc). The
tail call is a bit special, but nevertheless generic for each user and [very]
useful, so it makes sense to inherit from the array map and move the code there.

I don't really like that we start add new _special_-cased maps here into the
eBPF core code, it seems quite hacky. :( From your rather terse commit description
where you introduce the maps, I failed to see a detailed elaboration on this i.e.
why it cannot be abstracted any different?

> Patch 4/5 implement function bpf_perf_event_read() that get the selected
> hardware PMU conuter;
>
> Patch 5/5 give a simple example.
>
> Kaixu Xia (5):
> bpf: Add new bpf map type to store the pointer to struct perf_event
> bpf: Add function map->ops->map_traverse_elem() to traverse map elems
> bpf: Save the pointer to struct perf_event to map
> bpf: Implement function bpf_perf_event_read() that get the selected
> hardware PMU conuter
> samples/bpf: example of get selected PMU counter value
>
> include/linux/bpf.h | 6 +++
> include/linux/perf_event.h | 5 ++-
> include/uapi/linux/bpf.h | 3 ++
> kernel/bpf/arraymap.c | 110 +++++++++++++++++++++++++++++++++++++++++++++
> kernel/bpf/helpers.c | 42 +++++++++++++++++
> kernel/bpf/syscall.c | 26 +++++++++++
> kernel/events/core.c | 30 ++++++++++++-
> kernel/trace/bpf_trace.c | 2 +
> samples/bpf/Makefile | 4 ++
> samples/bpf/bpf_helpers.h | 2 +
> samples/bpf/tracex6_kern.c | 27 +++++++++++
> samples/bpf/tracex6_user.c | 67 +++++++++++++++++++++++++++
> 12 files changed, 321 insertions(+), 3 deletions(-)
> create mode 100644 samples/bpf/tracex6_kern.c
> create mode 100644 samples/bpf/tracex6_user.c
>