Re: [RFC/PATCHSET 0/7] perf record: Implement BPF sample filter (v1)
From: Namhyung Kim
Date: Tue Feb 14 2023 - 13:02:15 EST
Hi Ian,
On Tue, Feb 14, 2023 at 8:58 AM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> 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.
Well.. I actually never tried the BPF events myself :)
I think we can deprecate it and get rid of it once the perf trace
conversion is done.
>
> - 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.
For now, it's applied to the last event specified by '-e' before the fitter.
As it's local to the event, you should be able to use appropriate one
for each event. I didn't think about the metrics as it's for perf record
only.
>
> - 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.
Maybe.. but note that the hardcoded max is just for the verifier.
At runtime, it should stop after processing the actual number
of filter items only.
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
> >