On Mon, Feb 13, 2023 at 9:05 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> 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.
>
> 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:
>
> "bpf:" <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 'bpf: 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 'bpf: 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-v1' 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.
This is great! I wonder about related clean up:
- can we remove BPF events as this is a better feature?
- I believe BPF events are flaky, seldom used (with the exception
of the augmented syscalls for perf trace, which really should move to
a BPF skeleton as most people don't know how to use it) and they add a
bunch of complexity. A particular complexity I care about is that the
path separator forward slash ('/') is also the modifier separator for
events.
- what will happen with multiple events/metrics? Perhaps there should
be a way of listing filters so that each filter applies to the
appropriate event in the event list, like cgroups and -G. For metrics
we shuffle the list of events and so maybe the filters need some way
to specify which event they apply to.
- It feels like there should be some BPF way of overcoming the fixed
length number of filters so it is still bounded but not a hardcoded
number.
Thanks,
Ian
> Thanks,
> Namhyung
>
> Namhyung Kim (7):
> 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
>
> tools/perf/Documentation/perf-record.txt | 10 +-
> tools/perf/Makefile.perf | 2 +-
> tools/perf/builtin-record.c | 46 ++++--
> tools/perf/util/Build | 16 ++
> tools/perf/util/bpf-filter.c | 117 ++++++++++++++
> tools/perf/util/bpf-filter.h | 48 ++++++
> tools/perf/util/bpf-filter.l | 146 ++++++++++++++++++
> tools/perf/util/bpf-filter.y | 55 +++++++
> tools/perf/util/bpf_counter.c | 3 +-
> tools/perf/util/bpf_skel/sample-filter.h | 25 +++
> tools/perf/util/bpf_skel/sample_filter.bpf.c | 152 +++++++++++++++++++
> tools/perf/util/evsel.c | 2 +
> tools/perf/util/evsel.h | 7 +-
> tools/perf/util/parse-events.c | 4 +
> tools/perf/util/session.c | 3 +-
> 15 files changed, 615 insertions(+), 21 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: 37f322cd58d81a9d46456531281c908de9ef6e42
> --
> 2.39.1.581.gbfd45094c4-goog
>
