This patch doesn't apply on coresight next.
On Wed, Jun 09, 2021 at 04:04:20PM +0300, James Clark wrote:
Currently, timeless mode starts the decode on PERF_RECORD_EXIT, and non-timeless mode starts decoding on the fist PERF_RECORD_AUX record.
This can cause the "data has no samples!" error if the first PERF_RECORD_AUX record comes before the first (or any relevant) PERF_RECORD_MMAP2 record because the mmaps are required by the decoder to access the binary data.
This change pushes the start of non-timeless decoding to the very end of parsing the file. The PERF_RECORD_EXIT event can't be used because it might not exist in system-wide or snapshot modes.
I have not been able to find the exact cause for the events to be intermittently in the wrong order in the basic scenario:
perf record -e cs_etm/@tmc_etr0/u top
But it can be made to happen every time with the --delay option. This is because "enable_on_exec" is disabled, which causes tracing to start before the process to be launched is exec'd. For example:
perf record -e cs_etm/@tmc_etr0/u --delay=1 top perf report -D | grep 'AUX|MAP'
0 16714475632740 0x520 [0x40]: PERF_RECORD_AUX offset: 0 size: 0x30 flags: 0 [] 0 16714476494960 0x5d0 [0x40]: PERF_RECORD_AUX offset: 0x30 size: 0x30 flags: 0 [] 0 16714478208900 0x660 [0x40]: PERF_RECORD_AUX offset: 0x60 size: 0x30 flags: 0 [] 4294967295 16714478293340 0x700 [0x70]: PERF_RECORD_MMAP2 8712/8712: [0x557a460000(0x54000) @ 0 00:17 5329258 0]: r-xp /usr/bin/top 4294967295 16714478353020 0x770 [0x88]: PERF_RECORD_MMAP2 8712/8712: [0x7f86f72000(0x34000) @ 0 00:17 5214354 0]: r-xp /usr/lib/aarch64-linux-gnu/ld-2.31.so
Another scenario in which decoding from the first aux record fails is a workload that forks. Although the aux record comes after 'bash', it comes before 'top', which is what we are interested in. For example:
perf record -e cs_etm/@tmc_etr0/u -- bash -c top perf report -D | grep 'AUX|MAP'
4294967295 16853946421300 0x510 [0x70]: PERF_RECORD_MMAP2 8723/8723: [0x558f280000(0x142000) @ 0 00:17 5213953 0]: r-xp /usr/bin/bash 4294967295 16853946543560 0x580 [0x88]: PERF_RECORD_MMAP2 8723/8723: [0x7fbba6e000(0x34000) @ 0 00:17 5214354 0]: r-xp /usr/lib/aarch64-linux-gnu/ld-2.31.so 4294967295 16853946628420 0x608 [0x68]: PERF_RECORD_MMAP2 8723/8723: [0x7fbba9e000(0x1000) @ 0 00:00 0 0]: r-xp [vdso] 0 16853947067300 0x690 [0x40]: PERF_RECORD_AUX offset: 0 size: 0x3a60 flags: 0 [] ... 0 16853966602580 0x1758 [0x40]: PERF_RECORD_AUX offset: 0xc2470 size: 0x30 flags: 0 [] 4294967295 16853967119860 0x1818 [0x70]: PERF_RECORD_MMAP2 8723/8723: [0x5559e70000(0x54000) @ 0 00:17 5329258 0]: r-xp /usr/bin/top 4294967295 16853967181620 0x1888 [0x88]: PERF_RECORD_MMAP2 8723/8723: [0x7f9ed06000(0x34000) @ 0 00:17 5214354 0]: r-xp /usr/lib/aarch64-linux-gnu/ld-2.31.so 4294967295 16853967237180 0x1910 [0x68]: PERF_RECORD_MMAP2 8723/8723: [0x7f9ed36000(0x1000) @ 0 00:00 0 0]: r-xp [vdso]
A third scenario is when the majority of time is spent in a shared library that is not loaded at startup. For example a dynamically loaded plugin.
Testing
Testing was done by checking if any samples that are present in the old output are missing from the new output. Timestamps must be stripped out with awk because now they are set to the last AUX sample, rather than the first:
./perf script $4 | awk '!($4="")' > new.script ./perf-default script $4 | awk '!($4="")' > default.script comm -13 <(sort -u new.script) <(sort -u default.script)
Testing showed that the new output is a superset of the old. When lines appear in the comm output, it is not because they are missing but because [unknown] is now resolved to sensible locations. For example last putp branch here now resolves to libtinfo, so it's not missing from the output, but is actually improved:
Old: top 305 [001] 1 branches:uH: 402830 _init+0x30 (/usr/bin/top.procps) => 404a1c [unknown] (/usr/bin/top.procps) top 305 [001] 1 branches:uH: 404a20 [unknown] (/usr/bin/top.procps) => 402970 putp@plt+0x0 (/usr/bin/top.procps) top 305 [001] 1 branches:uH: 40297c putp@plt+0xc (/usr/bin/top.procps) => 0 [unknown] ([unknown]) New: top 305 [001] 1 branches:uH: 402830 _init+0x30 (/usr/bin/top.procps) => 404a1c [unknown] (/usr/bin/top.procps) top 305 [001] 1 branches:uH: 404a20 [unknown] (/usr/bin/top.procps) => 402970 putp@plt+0x0 (/usr/bin/top.procps) top 305 [001] 1 branches:uH: 40297c putp@plt+0xc (/usr/bin/top.procps) => 7f8ab39208 putp+0x0 (/lib/libtinfo.so.5.9)
In the following two modes, decoding now works and the "data has no samples!" error is not displayed any more:
perf record -e cs_etm/@tmc_etr0/u -- bash -c top perf record -e cs_etm/@tmc_etr0/u --delay=1 top
In snapshot mode, there is also an improvement to decoding. Previously samples for the 'kill' process that was used to send SIGUSR2 were completely missing, because the process hadn't started yet. But now there are additional samples present:
perf record -e cs_etm/@tmc_etr0/u --snapshot -a perf script
stress 19380 [003] 161627.938153: 1000000 instructions:uH: aaaabb612fb4 [unknown] (/usr/bin/stress) kill 19644 [000] 161627.938153: 1000000 instructions:uH: ffffae0ef210 [unknown] (/lib/aarch64-linux-gnu/ld-2.27.so) stress 19380 [003] 161627.938153: 1000000 instructions:uH: ffff9e754d40 random_r+0x20 (/lib/aarch64-linux-gnu/libc-2.27.so)Also tested was the round trip of 'perf inject' followed by 'perf report' which has the same differences and improvements.
Signed-off-by: James Clark james.clark@arm.com
tools/perf/util/cs-etm.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c index 57aea2c7fc77..ceed0b038796 100644 --- a/tools/perf/util/cs-etm.c +++ b/tools/perf/util/cs-etm.c @@ -2407,6 +2407,11 @@ static int cs_etm__process_event(struct perf_session *session, return err; }
- /*
* Don't wait for cs_etm__flush_events() in per-thread/timeless mode to start the decode. We* need the tid of the PERF_RECORD_EXIT event to assign to the synthesised samples because* ETM_OPT_CTXTID is not enabled.*/@@ -2424,7 +2429,6 @@ static int cs_etm__process_event(struct perf_session *session, * onwards. */ etm->latest_kernel_timestamp = sample_kernel_timestamp;
return cs_etm__process_queues(etm);return 0; -- 2.28.0