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

[Arnaldo Carvalho de Melo]

Em Tue, Feb 14, 2023 at 08:57:58AM -0800, Ian Rogers escreveu:
> On Mon, Feb 13, 2023 at 9:05 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> > 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>)*

bpf is the technology used for that, but this really is about filtering
by fields in the sample type, right? So perhaps we could remove that
"bpf:" part and simply do:

  sudo ./perf record -e cycles --filter 'period > 10000' -- ./perf test -w noploop

And notice that this requires this new mechanism and just use it? It
gets more compact and should be unambiguous for non-tracepoint events?

And for tracepoint events if we can use both mechanisms, then use the
tracepoint one since it requires less setup?

Perhaps use "sample_type.field" to disambiguate if we would like to get a
field from the sample_type and another in the tracepoint if both have
the same name?

And how difficult it would be to just accept the same syntax (or a
superset) of what is available for tracepoint filters? I.e. allow || as
well as &&.

Great stuff!

- Arnaldo

> > 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
> >

-- 

- Arnaldo