Re: [RFC/PATCHSET 0/7] perf record: Implement BPF sample filter (v1)

[Ian Rogers]

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
>