Subject: Re: [PATCH] perf cs-etm: Delay decode of non-timeless data until cs_etm__flush_events()

From: Mathieu Poirier <[email-masked]>
Date: Mon, 14 Jun 2021 09:54:42 -0600

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 <[email-masked]>
> ---
>  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.
> +	 */
>  	if (etm->timeless_decoding &&
>  	    event->header.type == PERF_RECORD_EXIT)
>  		return cs_etm__process_timeless_queues(etm,
> @@ -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);

This looks good to me but I'd like to have other people testing it.

Thanks,
Mathieu

>  	}
>  	return 0;
> -- 
> 2.28.0

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