[RFC/PATCHSET 0/8] perf record: Implement BPF sample filter (v2)

Namhyung Kim
2023-02-19T06:13:37Z

Hello,

There have been requests for more sophisticated perf event sample
filtering based on the sample data. Recently the kernel added BPF
programs can access perf sample data and this is the userspace part
to enable such a filtering.

This still has some rough edges and needs more improvements. But
I'd like to share the current work and get some feedback for the
directions and idea for further improvements.

v2 changes)
* fix build error with the misc field (Jiri)
* add a destructor for filter expr (Ian)
* remove 'bpf:' prefix (Arnaldo)
* add '||' operator

The kernel changes are in the tip.git tree (perf/core branch) for now.
perf record has --filter option to set filters on the last specified
event in the command line. It worked only for tracepoints and Intel
PT events so far. This patchset extends it to have 'bpf:' prefix in
order to enable the general sample filters using BPF for any events.

A new filter expression parser was added (using flex/bison) to process
the filter string. Right now, it only accepts very simple expressions
separated by comma. I'd like to keep the filter expression as simple
as possible.

It requires samples satisfy all the filter expressions otherwise it'd
drop the sample. IOW filter expressions are connected with logical AND
operations implicitly.

Essentially the BPF filter expression is:

<term> <operator> <value> (("," | "||") <term> <operator> <value>)*

The <term> can be one of:
ip, id, tid, pid, cpu, time, addr, period, txn, weight, phys_addr,
code_pgsz, data_pgsz, weight1, weight2, weight3, ins_lat, retire_lat,
p_stage_cyc, mem_op, mem_lvl, mem_snoop, mem_remote, mem_lock,
mem_dtlb, mem_blk, mem_hops

The <operator> can be one of:
==, !=, >, >=, <, <=, &

The <value> can be one of:
<number> (for any term)
na, load, store, pfetch, exec (for mem_op)
l1, l2, l3, l4, cxl, io, any_cache, lfb, ram, pmem (for mem_lvl)
na, none, hit, miss, hitm, fwd, peer (for mem_snoop)
remote (for mem_remote)
na, locked (for mem_locked)
na, l1_hit, l1_miss, l2_hit, l2_miss, any_hit, any_miss, walk, fault (for mem_dtlb)
na, by_data, by_addr (for mem_blk)
hops0, hops1, hops2, hops3 (for mem_hops)

I plan to improve it with range expressions like for ip or addr and it
should support symbols like the existing addr-filters. Also cgroup
should understand and convert cgroup names to IDs.

Let's take a look at some examples. The following is to profile a user
program on the command line. When the frequency mode is used, it starts
with a very small period (i.e. 1) and adjust it on every interrupt (NMI)
to catch up the given frequency.

$ ./perf record -- ./perf test -w noploop
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.263 MB perf.data (4006 samples) ]

$ ./perf script -F pid,period,event,ip,sym | head
36695 1 cycles: ffffffffbab12ddd perf_event_exec
36695 1 cycles: ffffffffbab12ddd perf_event_exec
36695 5 cycles: ffffffffbab12ddd perf_event_exec
36695 46 cycles: ffffffffbab12de5 perf_event_exec
36695 1163 cycles: ffffffffba80a0eb x86_pmu_disable_all
36695 1304 cycles: ffffffffbaa19507 __hrtimer_get_next_event
36695 8143 cycles: ffffffffbaa186f9 __run_timers
36695 69040 cycles: ffffffffbaa0c393 rcu_segcblist_ready_cbs
36695 355117 cycles: 4b0da4 noploop
36695 321861 cycles: 4b0da4 noploop

If you want to skip the first few samples that have small periods, you
can do like this (note it requires root due to BPF).

$ sudo ./perf record -e cycles --filter 'period > 10000' -- ./perf test -w noploop
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.262 MB perf.data (3990 samples) ]

$ sudo ./perf script -F pid,period,event,ip,sym | head
39524 58253 cycles: ffffffffba97dac0 update_rq_clock
39524 232657 cycles: 4b0da2 noploop
39524 210981 cycles: 4b0da2 noploop
39524 282882 cycles: 4b0da4 noploop
39524 392180 cycles: 4b0da4 noploop
39524 456058 cycles: 4b0da4 noploop
39524 415196 cycles: 4b0da2 noploop
39524 462721 cycles: 4b0da4 noploop
39524 526272 cycles: 4b0da2 noploop
39524 565569 cycles: 4b0da4 noploop

Maybe more useful example is when it deals with precise memory events.
On AMD processors with IBS, you can filter only memory load with L1
dTLB is missed like below.

$ sudo ./perf record -ad -e ibs_op//p \
> --filter 'mem_op == load, mem_dtlb > l1_hit' sleep 1
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 1.338 MB perf.data (15 samples) ]

$ sudo ./perf script -F data_src | head
51080242 |OP LOAD|LVL LFB/MAB hit|SNP N/A|TLB L2 miss|LCK N/A|BLK N/A
49080142 |OP LOAD|LVL L1 hit|SNP N/A|TLB L2 hit|LCK N/A|BLK N/A
51080242 |OP LOAD|LVL LFB/MAB hit|SNP N/A|TLB L2 miss|LCK N/A|BLK N/A
51080242 |OP LOAD|LVL LFB/MAB hit|SNP N/A|TLB L2 miss|LCK N/A|BLK N/A
51088842 |OP LOAD|LVL L3 or Remote Cache (1 hop) hit|SNP N/A|TLB L2 miss|LCK N/A|BLK N/A
51080242 |OP LOAD|LVL LFB/MAB hit|SNP N/A|TLB L2 miss|LCK N/A|BLK N/A
51080242 |OP LOAD|LVL LFB/MAB hit|SNP N/A|TLB L2 miss|LCK N/A|BLK N/A
51080242 |OP LOAD|LVL LFB/MAB hit|SNP N/A|TLB L2 miss|LCK N/A|BLK N/A
49080442 |OP LOAD|LVL L2 hit|SNP N/A|TLB L2 hit|LCK N/A|BLK N/A
51080242 |OP LOAD|LVL LFB/MAB hit|SNP N/A|TLB L2 miss|LCK N/A|BLK N/A

You can also check the number of dropped samples in LOST_SAMPLES events
using perf report --stat command.

$ sudo ./perf report --stat

Aggregated stats:
TOTAL events: 16066
MMAP events: 22 ( 0.1%)
COMM events: 4166 (25.9%)
EXIT events: 1 ( 0.0%)
THROTTLE events: 816 ( 5.1%)
UNTHROTTLE events: 613 ( 3.8%)
FORK events: 4165 (25.9%)
SAMPLE events: 15 ( 0.1%)
MMAP2 events: 6133 (38.2%)
LOST_SAMPLES events: 1 ( 0.0%)
KSYMBOL events: 69 ( 0.4%)
BPF_EVENT events: 57 ( 0.4%)
FINISHED_ROUND events: 3 ( 0.0%)
ID_INDEX events: 1 ( 0.0%)
THREAD_MAP events: 1 ( 0.0%)
CPU_MAP events: 1 ( 0.0%)
TIME_CONV events: 1 ( 0.0%)
FINISHED_INIT events: 1 ( 0.0%)
ibs_op//p stats:
SAMPLE events: 15
LOST_SAMPLES events: 3991

Note that the total aggregated stats show 1 LOST_SAMPLES event but
per event stats show 3991 events because it's the actual number of
dropped samples while the aggregated stats has the number of record.
Maybe we need to change the per-event stats to 'LOST_SAMPLES count'
to avoid the confusion.

The code is available at 'perf/bpf-filter-v2' branch in my tree.

git://git.kernel.org/pub/scm/linux/kernel/git/namhyung/linux-perf.git

Again, you need tip/perf/core kernel for this to work.
Any feedback is welcome.

Thanks,
Namhyung

Namhyung Kim (8):
perf bpf filter: Introduce basic BPF filter expression
perf bpf filter: Implement event sample filtering
perf record: Add BPF event filter support
perf record: Record dropped sample count
perf bpf filter: Add 'pid' sample data support
perf bpf filter: Add more weight sample data support
perf bpf filter: Add data_src sample data support
perf bpf filter: Add logical OR operator

tools/lib/perf/include/perf/event.h | 2 +
tools/perf/Documentation/perf-record.txt | 15 +-
tools/perf/Makefile.perf | 2 +-
tools/perf/builtin-record.c | 46 ++++--
tools/perf/util/Build | 16 ++
tools/perf/util/bpf-filter.c | 142 ++++++++++++++++
tools/perf/util/bpf-filter.h | 49 ++++++
tools/perf/util/bpf-filter.l | 147 +++++++++++++++++
tools/perf/util/bpf-filter.y | 77 +++++++++
tools/perf/util/bpf_counter.c | 3 +-
tools/perf/util/bpf_skel/sample-filter.h | 27 +++
tools/perf/util/bpf_skel/sample_filter.bpf.c | 164 +++++++++++++++++++
tools/perf/util/evsel.c | 2 +
tools/perf/util/evsel.h | 7 +-
tools/perf/util/parse-events.c | 8 +-
tools/perf/util/session.c | 3 +-
16 files changed, 682 insertions(+), 28 deletions(-)
create mode 100644 tools/perf/util/bpf-filter.c
create mode 100644 tools/perf/util/bpf-filter.h
create mode 100644 tools/perf/util/bpf-filter.l
create mode 100644 tools/perf/util/bpf-filter.y
create mode 100644 tools/perf/util/bpf_skel/sample-filter.h
create mode 100644 tools/perf/util/bpf_skel/sample_filter.bpf.c

base-commit: f9fa0778ee7349a9aa3d2ea10e9f2ab843a0b44e
--
2.39.2.637.g21b0678d19-goog