[RFC PATCH 0/4] perf tools: Use the new ability of eBPF programs to access hardware PMU counter

From: Kaixu Xia
Date: Thu Aug 27 2015 - 06:43:51 EST

According to the discussions on this subject https://lkml.org/lkml/2015/5/27/1027,
we want to give eBPF programs the ability to access hardware PMU counter
and use this ability with perf.

Now the kernel side patch set 'bpf: Introduce the new ability of eBPF
programs to access hardware PMU counter' has been applied and can be
found in the net-next tree.

ffe8690c85b8 ("perf: add the necessary core perf APIs when accessing events counters in eBPF programs")
2a36f0b92eb6 ("bpf: Make the bpf_prog_array_map more generic")
ea317b267e9d ("bpf: Add new bpf map type to store the pointer to struct perf_event")
35578d798400 ("bpf: Implement function bpf_perf_event_read() that get the selected hardware PMU conuter")
47efb30274cb ("samples/bpf: example of get selected PMU counter value")

According to the design plan, we still need the perf side code.
This patch set based on Wang Nan's patches (perf tools: filtering events
using eBPF programs).
(git://git.kernel.org/pub/scm/linux/kernel/git/pi3orama/linux tags/perf-ebpf-for-acme-20150821)
The kernel side patch set above also need to be mergerd if you want to
test this patch set.

An example is pasted at the bottom of this cover letter. In that example,
we can get the cpu_cycles and exception taken in sys_write.

$ cat /sys/kernel/debug/tracing/trace_pipe
$ ./perf record --event perf-bpf.o ls
...
cat-1653 [003] d..1 88174.613854: : ente: CPU-3 cyc:48746333 exc:84
cat-1653 [003] d..2 88174.613861: : exit: CPU-3 cyc:48756041 exc:84
cat-1653 [003] d..1 88174.613872: : ente: CPU-3 cyc:48771199 exc:86
cat-1653 [003] d..2 88174.613879: : exit: CPU-3 cyc:48780448 exc:86
cat-1678 [003] d..1 88174.615001: : ente: CPU-3 cyc:50293479 exc:93
sshd-1669 [000] d..1 88174.615199: : ente: CPU-0 cyc:44402694 exc:51
sshd-1669 [000] d..2 88174.615283: : exit: CPU-0 cyc:44517335 exc:51
ls-1680 [003] d..1 88174.620260: : ente: CPU-3 cyc:57281750 exc:241
sshd-1669 [000] d..1 88174.620474: : ente: CPU-0 cyc:44998837 exc:69
sshd-1669 [000] d..2 88174.620549: : exit: CPU-0 cyc:45101855 exc:69
sshd-1669 [000] d..1 88174.620608: : ente: CPU-0 cyc:45181848 exc:77
sshd-1669 [000] d..2 88174.620709: : exit: CPU-0 cyc:45317439 exc:78
sshd-1669 [000] d..1 88174.620801: : ente: CPU-0 cyc:45441321 exc:87
sshd-1669 [000] d..2 88174.620856: : exit: CPU-0 cyc:45515882 exc:87
...

Limitation of this patch set: The eBPF programs can only create and access
the perf events depend on CPUs and can not do that depend on PID.

The detail of patches is as follow:

Patch 1/4 introduces bpf_update_elem() and perf_event_open() in
introduces bpf_update_elem() and perf_event_open(). We can store
the pointers to struct perf_event to maps;

Patch 2/4 collects BPF_MAP_TYPE_PERF_EVENT_ARRAY map definitions
from 'maps' section and get the event & map match;

Patch 3/4 saves the perf event fds from "maps" sections to
'struct bpf_object'. So we can enable/disable these perf events
at the appropriate time;

Patch 4/4 enable/disable the perf events stored in 'struct bpf_object';

-------- EXAMPL --------
----- perf-bpf.c -----

struct perf_event_map_def SEC("maps") my_cycles_map = {
.map_def = {
.type = BPF_MAP_TYPE_PERF_EVENT_ARRAY,
.key_size = sizeof(int),
.value_size = sizeof(u32),
.max_entries = 32,
},
.attr = {
.freq = 0,
.inherit = 0,
.sample_period = 0x7fffffffffffffffULL,
.type = PERF_TYPE_HARDWARE,
.read_format = 0,
.sample_type = 0,
.config = 0,/* PMU: cycles */
},
};

struct perf_event_map_def SEC("maps") my_exception_map = {
.map_def = {
.type = BPF_MAP_TYPE_PERF_EVENT_ARRAY,
.key_size = sizeof(int),
.value_size = sizeof(u32),
.max_entries = 32,
},
.attr = {
.freq = 0,
.inherit = 0,
.sample_period = 0x7fffffffffffffffULL,
.type = PERF_TYPE_RAW,
.read_format = 0,
.sample_type = 0,
.config = 0x09,/* PMU: exception */
},
};

SEC("ente=sys_write")
int bpf_prog_1(struct pt_regs *ctx)
{
u64 count_cycles, count_exception;
u32 key = bpf_get_smp_processor_id();
char fmt[] = "ente: CPU-%d cyc:%llu exc:%llu\n";

count_cycles = bpf_perf_event_read(&my_cycles_map, key);
count_exception = bpf_perf_event_read(&my_exception_map, key);
bpf_trace_printk(fmt, sizeof(fmt), key, count_cycles, count_exception);

return 0;
}

SEC("exit=sys_write%return")
int bpf_prog_2(struct pt_regs *ctx)
{
u64 count_cycles, count_exception;
u32 key = bpf_get_smp_processor_id();
char fmt[] = "exit: CPU-%d cyc:%llu exc:%llu\n";

count_cycles = bpf_perf_event_read(&my_cycles_map, key);
count_exception = bpf_perf_event_read(&my_exception_map, key);
bpf_trace_printk(fmt, sizeof(fmt), key, count_cycles, count_exception);

return 0;
}

Kaixu Xia (4):
bpf tools: Add bpf_update_elem() and perf_event_open() for common bpf
operations
bpf tools: Collect BPF_MAP_TYPE_PERF_EVENT_ARRAY map definitions from
'maps' section
bpf tools: Save the perf event fds from "maps" sections to 'struct
bpf_object'
bpf tools: Enable/disable the perf events stored in 'struct
bpf_object'

tools/lib/bpf/bpf.c | 34 +++++++++++++
tools/lib/bpf/bpf.h | 4 ++
tools/lib/bpf/libbpf.c | 130 +++++++++++++++++++++++++++++++++++++++++++------
tools/lib/bpf/libbpf.h | 13 +++++
4 files changed, 166 insertions(+), 15 deletions(-)

--
1.8.3.4

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