[PATCH v1 0/2]: perf: reduce data loss when profiling highly parallel CPU bound workloads

Alexey Budankov
2018-08-23T10:26:32Z

Currently in record mode the tool implements trace writing serially.
The algorithm loops over mapped per-cpu data buffers and stores ready
data chunks into a trace file using write() system call.

At some circumstances the kernel may lack free space in a buffer
because the other buffer's half is not yet written to disk due to
some other buffer's data writing by the tool at the moment.

Thus serial trace writing implementation may cause the kernel
to loose profiling data and that is what observed when profiling
highly parallel CPU bound workloads on machines with big number
of cores.

Experiment with profiling matrix multiplication code executing 128
threads on Intel Xeon Phi (KNM) with 272 cores, like below,
demonstrates data loss metrics value of 98%:

/usr/bin/time perf record -o /tmp/perf-ser.data -a -N -B -T -R -g \
--call-graph dwarf,1024 --user-regs=IP,SP,BP \
--switch-events -e cycles,instructions,ref-cycles,software/period=1,name=cs,config=0x3/Duk -- \
matrix.gcc

Data loss metrics is the ratio lost_time/elapsed_time where
lost_time is the sum of time intervals containing PERF_RECORD_LOST
records and elapsed_time is the elapsed application run time
under profiling.

Applying asynchronous trace streaming thru Posix AIO API
http://man7.org/linux/man-pages/man7/aio.7.html lowers data loss
metrics value providing ~25% improvement in average.

Sanity testing:

tools/perf/perf test
1: vmlinux symtab matches kallsyms : Ok
2: Detect openat syscall event : Ok
3: Detect openat syscall event on all cpus : Ok
4: Read samples using the mmap interface : Ok
5: Test data source output : Ok
6: Parse event definition strings : Ok
7: Simple expression parser : Ok
8: PERF_RECORD_* events & perf_sample fields : Ok
9: Parse perf pmu format : Ok
10: DSO data read : Ok
11: DSO data cache : Ok
12: DSO data reopen : Ok
13: Roundtrip evsel->name : Ok
14: Parse sched tracepoints fields : Ok
15: syscalls:sys_enter_openat event fields : Ok
16: Setup struct perf_event_attr : Skip
17: Match and link multiple hists : Ok
18: 'import perf' in python : FAILED!
19: Breakpoint overflow signal handler : Ok
20: Breakpoint overflow sampling : Ok
21: Breakpoint accounting : Ok
22: Number of exit events of a simple workload : Ok
23: Software clock events period values : Ok
24: Object code reading : Ok
25: Sample parsing : Ok
26: Use a dummy software event to keep tracking : Ok
27: Parse with no sample_id_all bit set : Ok
28: Filter hist entries : Ok
29: Lookup mmap thread : Ok
30: Share thread mg : Ok
31: Sort output of hist entries : Ok
32: Cumulate child hist entries : Ok
33: Track with sched_switch : Ok
34: Filter fds with revents mask in a fdarray : Ok
35: Add fd to a fdarray, making it autogrow : Ok
36: kmod_path__parse : Ok
37: Thread map : Ok
38: LLVM search and compile :
38.1: Basic BPF llvm compile : Skip
38.2: kbuild searching : Skip
38.3: Compile source for BPF prologue generation : Skip
38.4: Compile source for BPF relocation : Skip
39: Session topology : Ok
40: BPF filter :
40.1: Basic BPF filtering : Skip
40.2: BPF pinning : Skip
40.3: BPF prologue generation : Skip
40.4: BPF relocation checker : Skip
41: Synthesize thread map : Ok
42: Remove thread map : Ok
43: Synthesize cpu map : Ok
44: Synthesize stat config : Ok
45: Synthesize stat : Ok
46: Synthesize stat round : Ok
47: Synthesize attr update : Ok
48: Event times : Ok
49: Read backward ring buffer : Ok
50: Print cpu map : Ok
51: Probe SDT events : Ok
52: is_printable_array : Ok
53: Print bitmap : Ok
54: perf hooks : Ok
55: builtin clang support : Skip (not compiled in)
56: unit_number__scnprintf : Ok
57: mem2node : Ok
58: x86 rdpmc : Ok
59: Convert perf time to TSC : Ok
60: DWARF unwind : Ok
61: x86 instruction decoder - new instructions : Ok
62: Use vfs_getname probe to get syscall args filenames : Skip
63: Add vfs_getname probe to get syscall args filenames : Skip
64: Check open filename arg using perf trace + vfs_getname: Skip
65: probe libc's inet_pton & backtrace it with ping : FAILED!

---
Alexey Budankov (2):
perf util: map data buffer for preserving collected data
perf record: enable asynchronous trace writing

tools/perf/builtin-record.c | 113 +++++++++++++++++++++++++++++++++++++++++---
tools/perf/util/evlist.c | 9 ++++
tools/perf/util/evlist.h | 2 +
tools/perf/util/mmap.c | 35 +++++++++-----
tools/perf/util/mmap.h | 5 +-
5 files changed, 144 insertions(+), 20 deletions(-)