Thx Mathieu, if I'm running my test on one cpu (taskset -c 1 ./test.run) it works as expected and described by you. The babeltrace flag would be a good solution! Do you want me to file a bug/enhancement on this?
Regards Anders Wallin On Sat, Nov 15, 2014 at 6:59 PM, Mathieu Desnoyers < [email protected]> wrote: > ------------------------------ > > *From: *"Anders Wallin" <[email protected]> > *To: *[email protected] > *Sent: *Friday, November 14, 2014 8:28:18 AM > *Subject: *[lttng-dev] snapshot data corrupted after the output buffer > is filled > > Hi, > > it looks like snapshot events became faulty after the output max size is > reached. > The next snapshot get mixed events from different buffers. I have tested > it on 2.3, 2.4 and 2.5 > and it's in all of them, but it hits earlier in 2.5 due to the problem > reported in > http://lists.lttng.org/pipermail/lttng-dev/2014-November/023773.html > > There should never be more then 2 HUNKS in a diff between 2 consecutive > snapshots, here I got > 11 as the worst case. > > Hi Anders, > > This is not a bug, rather just per-cpu buffering at work.It does have some > rather non-obvious consequences in terms of snapshot event flow. > > To understand this, we need to keep in mind that LTTng does everything > in per-cpu buffers, and that overwrite of oldest events in buffer full > conditions > happens per-cpu. > > The use-case you show below really makes sense when you start to consider > on which CPU (and thus per-cpu buffer) the application is running. > Initially, > the first app runs on CPU 6. As long as the following applications run on > other cpus, we keep the same first event. Then when enough events are > written in cpu 6 buffer to overwrite the first event, we see a first event > change. > Then, depending on the scheduling of applications, older parts of per-cpu > buffers are overwritten, and others are not (if there are less events in > those > buffer). This means some per-cpu buffers will have events going further > back > in time compared to other higher-throughput CPUs. > > This is why you see more than 2 hunks in your diff. Babeltrace merges those > per-cpu buffers back into a single text output, and the fact that some > events > have been overwritten for a time-range for high-throughput buffers, but not > for low-throughput buffers, make it appear as if events are missing in the > stream if your model is a single system-wide buffer, but the missing events > make sense if you see it as a merge of per-cpu buffers. > > Our current behavior is to gather all available data when taking a > snapshot, > and babeltrace behavior is to show all events available in the streams. We > could perhaps consider a different babeltrace behavior (a flag) that would > allow it to only show the events for the intersection of time range of all > streams, based on the packet header timestamp begin/end. This would > take care of hiding events that happen in low-throughput streams when > there is no intersection with snapshotted high-throughput streams. > > And perhaps this should be the default ? > > Thanks! > > Mathieu > > > TESTCASE; > > tracetest code: > --------------------------------------------------------------- > #include <stdio.h> > #include "tracetest-tp.h" > > int main(int argc, char* argv[]) > { > int i; > for (i = 0; i < 1000; i++) { > tracepoint(tracetest, first_tp, i, "test"); > } > return 0; > } > --------------------------------------------------------------- > > test script: > --------------------------------------------------------------- > #!/bin/bash > : ${NO_OF_SUBBUF:=6} > : ${SUBBUF_SIZE:=4096} > : ${KERNEL_TRACE:=NO} > : ${LOOPS:=10} > > SESSION=test > CHANNEL1=ch1 > CHANNEL2=ch2 > > CH2_SUBBUF_SIZE=$(($SUBBUF_SIZE*$NO_OF_SUBBUF)) > > > LTTNG='lttng -n' > killall lttng-consumerd > /dev/null 2>&1 > killall lttng-sessiond > /dev/null 2>&1 > if [ -z $DEBUG ]; then > lttng-sessiond --no-kernel -d > else > lttng-sessiond -v --no-kernel --verbose-consumer -b > fi > sleep 1 > > if [ ! -z $DEBUG ]; then > export LTTNG_UST_DEBUG=1 > fi > > rm -rf $HOME/lttng-traces > rm snapshot/*.bt > > $LTTNG --version > > $LTTNG create $SESSION --snapshot > $LTTNG enable-channel -u --subbuf-size $SUBBUF_SIZE --num-subbuf > $NO_OF_SUBBUF --overwrite $CHANNEL1 > $LTTNG enable-channel -u --subbuf-size $CH2_SUBBUF_SIZE --num-subbuf 2 > --overwrite $CHANNEL2 > $LTTNG enable-event -u -c $CHANNEL1 "*" > $LTTNG start > $LTTNG list $SESSION > > mkdir -p snapshot/ > > h=1 > for i in $(seq 1 $LOOPS); do > $APP > $LTTNG snapshot record > # print no of events and first and last event > > > last=$(ls -1drt $HOME/lttng-traces/$SESSION*/* | tail -1) > babeltrace $last > snapshot/$i.bt > cat snapshot/$i.bt | wc -l > cat snapshot/$i.bt | head -1 > cat snapshot/$i.bt | tail -1 > diff -u snapshot/$i.bt snapshot/$h.bt > snapshot/$i_$h.diff > cat snapshot/$i_$h.diff | grep "@@" | wc -l | xargs echo "NUMBER OF > DIFF HUNKS = " > h=$i > done > > $LTTNG stop > $LTTNG destroy $SESSION > > --------------------------------------------------------------- > > result from run (removed some output lines for readability) > --------------------------------------------------------------- > lttng (LTTng Trace Control) 2.4.2 - Époque Opaque > Snapshot mode set. Every channel enabled for that session will be set in > overwrite mode and mmap output. > Warning: The number of subbuffers (6) is rounded to the next power of 2 (8) > UST channel ch1 enabled for session test > Warning: The subbuf size (24576) is rounded to the next power of 2 (32768) > UST channel ch2 enabled for session test > UST event * created in channel ch1 > Tracing started for session test > Tracing session test: [active snapshot] > Trace path: > > === Domain: UST global === > > Buffer type: per UID > > Channels: > ------------- > - ch1: [enabled] > > Attributes: > overwrite mode: 1 > subbufers size: 4096 > number of subbufers: 8 > switch timer interval: 0 > read timer interval: 0 > output: mmap() > > Events: > * (type: tracepoint) [enabled] > - ch2: [enabled] > > Attributes: > overwrite mode: 1 > subbufers size: 32768 > number of subbufers: 2 > switch timer interval: 0 > read timer interval: 0 > output: mmap() > > Events: > None > > Snapshot recorded successfully for session test > 1011 > [07:09:43.278933559] (+?.?????????) 07f8cf84a38f > ust_baddr_statedump:soinfo: { cpu_id = 6 }, { baddr = 0x7FFF645EE000, > sopath = "[vdso]", size = 0, mtime = -1 } > [07:09:43.279975100] (+0.000000975) 07f8cf84a38f tracetest:first_tp: { > cpu_id = 5 }, { my_string_field = "test", my_integer_field = 999 } > NUMBER OF DIFF HUNKS = 0 > Snapshot recorded successfully for session test > 2022 > [07:09:43.278933559] (+?.?????????) 07f8cf84a38f > ust_baddr_statedump:soinfo: { cpu_id = 6 }, { baddr = 0x7FFF645EE000, > sopath = "[vdso]", size = 0, mtime = -1 } > [07:09:43.337703508] (+0.000001238) 07f8cf84a38f tracetest:first_tp: { > cpu_id = 4 }, { my_string_field = "test", my_integer_field = 999 } > NUMBER OF DIFF HUNKS = 1 > Snapshot recorded successfully for session test > 3033 > [07:09:43.278933559] (+?.?????????) 07f8cf84a38f > ust_baddr_statedump:soinfo: { cpu_id = 6 }, { baddr = 0x7FFF645EE000, > sopath = "[vdso]", size = 0, mtime = -1 } > [07:09:43.432827051] (+0.000000308) 07f8cf84a38f tracetest:first_tp: { > cpu_id = 6 }, { my_string_field = "test", my_integer_field = 999 } > NUMBER OF DIFF HUNKS = 1 > Snapshot recorded successfully for session test > 4044 > [07:09:43.278933559] (+?.?????????) 07f8cf84a38f > ust_baddr_statedump:soinfo: { cpu_id = 6 }, { baddr = 0x7FFF645EE000, > sopath = "[vdso]", size = 0, mtime = -1 } > [07:09:43.475784825] (+0.000000501) 07f8cf84a38f tracetest:first_tp: { > cpu_id = 5 }, { my_string_field = "test", my_integer_field = 999 } > NUMBER OF DIFF HUNKS = 1 > Snapshot recorded successfully for session test > 4735 > [07:09:43.279031320] (+?.?????????) 07f8cf84a38f tracetest:first_tp: { > cpu_id = 5 }, { my_string_field = "test", my_integer_field = 0 } > [07:09:43.549945106] (+0.000000517) 07f8cf84a38f tracetest:first_tp: { > cpu_id = 6 }, { my_string_field = "test", my_integer_field = 999 } > NUMBER OF DIFF HUNKS = 12 > Snapshot recorded successfully for session test > 4437 > [07:09:43.335916076] (+?.?????????) 07f8cf84a38f > ust_baddr_statedump:soinfo: { cpu_id = 2 }, { baddr = 0x7FFFAD1FE000, > sopath = "[vdso]", size = 0, mtime = -1 } > [07:09:43.654229030] (+0.000000436) 07f8cf84a38f tracetest:first_tp: { > cpu_id = 5 }, { my_string_field = "test", my_integer_field = 999 } > NUMBER OF DIFF HUNKS = 3 > Snapshot recorded successfully for session test > 4746 > [07:09:43.335916076] (+?.?????????) 07f8cf84a38f > ust_baddr_statedump:soinfo: { cpu_id = 2 }, { baddr = 0x7FFFAD1FE000, > sopath = "[vdso]", size = 0, mtime = -1 } > [07:09:43.747509349] (+0.000000502) 07f8cf84a38f tracetest:first_tp: { > cpu_id = 6 }, { my_string_field = "test", my_integer_field = 999 } > NUMBER OF DIFF HUNKS = 11 > Snapshot recorded successfully for session test > 5757 > [07:09:43.335916076] (+?.?????????) 07f8cf84a38f > ust_baddr_statedump:soinfo: { cpu_id = 2 }, { baddr = 0x7FFFAD1FE000, > sopath = "[vdso]", size = 0, mtime = -1 } > [07:09:43.814938128] (+0.000001371) 07f8cf84a38f tracetest:first_tp: { > cpu_id = 4 }, { my_string_field = "test", my_integer_field = 999 } > NUMBER OF DIFF HUNKS = 1 > Snapshot recorded successfully for session test > 6066 > [07:09:43.335916076] (+?.?????????) 07f8cf84a38f > ust_baddr_statedump:soinfo: { cpu_id = 2 }, { baddr = 0x7FFFAD1FE000, > sopath = "[vdso]", size = 0, mtime = -1 } > [07:09:43.919112586] (+0.000000990) 07f8cf84a38f tracetest:first_tp: { > cpu_id = 6 }, { my_string_field = "test", my_integer_field = 999 } > NUMBER OF DIFF HUNKS = 11 > Snapshot recorded successfully for session test > 6077 > [07:09:43.335916076] (+?.?????????) 07f8cf84a38f > ust_baddr_statedump:soinfo: { cpu_id = 2 }, { baddr = 0x7FFFAD1FE000, > sopath = "[vdso]", size = 0, mtime = -1 } > [07:09:44.004234933] (+0.000001213) 07f8cf84a38f tracetest:first_tp: { > cpu_id = 6 }, { my_string_field = "test", my_integer_field = 999 } > NUMBER OF DIFF HUNKS = 2 > Waiting for data availability > Tracing stopped for session test > Session test destroyed > --------------------------------------------------------------- > > Regards > > Anders Wallin > > _______________________________________________ > lttng-dev mailing list > [email protected] > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev > > > > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com >
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