Messages in this thread | | | Date | Thu, 1 Jul 2021 21:34:04 +0800 | From | Leo Yan <> | Subject | Re: [PATCH] perf cs-etm: Delay decode of non-timeless data until cs_etm__flush_events() |
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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>
Excellent work, James! I learned several things from it :)
I went through the testing cases one by one, have verified all elaborated cases (but I don't verify injection cases):
Reviewed-by: Leo Yan <leo.yan@linaro.org> Tested-by: Leo Yan <leo.yan@linaro.org>
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