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    SubjectRe: [PATCH 00/14] [GIT PULL][v2.6.39] tracing/filter: More robust filtering

    hi Steve,

    * Steven Rostedt <rostedt@goodmis.org> wrote:

    > Steven Rostedt (14):
    > tracing/filter: Have no filter return a match
    > tracing/filter: Move OR and AND logic out of fn() method
    > tracing/filter: Dynamically allocate preds
    > tracing/filter: Call synchronize_sched() just once for system filters
    > tracing/filter: Allocate the preds in an array
    > tracing/filter: Free pred array on disabling of filter
    > tracing/filter: Use a tree instead of stack for filter_match_preds()
    > tracing/filter: Optimize short ciruit check
    > tracing/filter: Check the created pred tree
    > tracing/filter: Optimize filter by folding the tree
    > tracing/filter: Move MAX_FILTER_PRED to local tracing directory
    > tracing/filter: Increase the max preds to 2^14
    > tracing/filter: Swap entire filter of events
    > tracing/filter: Remove synchronize_sched() from __alloc_preds()

    I finally got around testing the various tracing filter features we have.

    Here is what i've done. Firstly, i have put my 'naive but curious user trying to
    make use of filters' hat on.

    I did:

    perf list | grep Tracepoint | less

    to get a list of tracepoints.

    #
    # Btw., unrelated feature request, it would be nice if the following shorcut did the
    # obvious thing:
    #
    # perf list Tracepoint
    #

    I picked one of the interesting looking tracepoints:

    syscalls:sys_enter_close [Tracepoint event]

    first roadblock:

    I had absolutely no idea how to proceed from here any further. I knew it from 'perf
    list --help' that I could stick 'syscalls:sys_enter_close' into -e expressions, but
    i had no idea how to utilize filter expressions at all.

    I could stick it into perf stat though, and it gave me:

    aldebaran:~> perf stat -a -e syscalls:sys_enter_close sleep 1

    Performance counter stats for 'sleep 1':

    12,240 syscalls:sys_enter_close

    1.004366276 seconds time elapsed

    Which is fine enough and just about right. Trying something more complex like
    'firefox' seems to mis-count the number of close() syscalls though, when compared
    to 'strace -c':

    aldebaran:~> strace -f -q -c firefox 2>&1 | head
    Error: cannot open display: :0
    % time seconds usecs/call calls errors syscall
    ...
    32.35 0.000011 0 271 4 close
    ...

    aldebaran:~> perf stat -e syscalls:sys_enter_close firefox
    Error: cannot open display: :0

    Performance counter stats for 'firefox':

    146 syscalls:sys_enter_close

    0.048227674 seconds time elapsed

    Note the 271 calls by strace, versus 146 calls by perf. One of them must be wrong -
    or they are counting different things.

    Using the syscalls:sys_enter_close event to filter i had to put my kernel hacker hat
    on and do:

    aldebaran:~> cat /debug/tracing/events/syscalls/sys_enter_close/format
    name: sys_enter_close
    ID: 404
    format:
    field:unsigned short common_type; offset:0; size:2; signed:0;
    field:unsigned char common_flags; offset:2; size:1; signed:0;
    field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
    field:int common_pid; offset:4; size:4; signed:1;
    field:int common_lock_depth; offset:8; size:4; signed:1;

    field:int nr; offset:12; size:4; signed:1;
    field:unsigned int fd; offset:16; size:8; signed:0;

    print fmt: "fd: 0x%08lx", ((unsigned long)(REC->fd))

    And putting my kernel tracing hacker hat on i knew that the only interesting piece
    of information for a filter would be the 'fd' word.

    Putting my naive user hat back on and knowing the 'fd' trick, i tried to filter out
    all close(10) calls from a 'hackbench' run. I knew that they existed:

    aldebaran:~> strace -f ./hackbench 1 2>&1 | grep 'close(10)'
    [pid 15277] close(10) = 0

    So i first tried to use them in 'perf stat':

    aldebaran:~> perf stat -e syscalls:sys_enter_close --filter 'fd == 10' ./hackbench 1
    Error: unknown option `filter'

    Ouch, no go - how come this does not work for perf stat? It makes quite a bit of
    sense to use it there as well.

    Anyway, knowing that it might work for 'perf record', i tried it there:

    aldebaran:~> perf record -e syscalls:sys_enter_close --filter 'fd == 10' ./hackbench 1
    Time: 0.079
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.007 MB perf.data (~325 samples) ]

    Cool i thought, it has 325 samples, success!

    Lets look at it via 'perf report' i thought:

    aldebaran:~> perf report
    aldebaran:~>

    ouch. The TUI flickered something which went away immediately (feature request:
    don't do that - at least try to mumble something about non-existent data or so).

    As a naive user i had no idea whether this was a perf report bug or whether i did
    something wrong.

    As a kernel hacker i knew that this meant that the events in the perf.data werent
    enough to display anything. As a kernel tracing hacker i knew i had to look at 'perf
    report -D' output:

    aldebaran:~> perf report -D | tail -15
    TOTAL events: 107
    MMAP events: 59
    LOST events: 0
    COMM events: 2
    EXIT events: 3
    THROTTLE events: 0
    UNTHROTTLE events: 0
    FORK events: 40
    READ events: 0
    SAMPLE events: 0
    ATTR events: 0
    EVENT_TYPE events: 0
    TRACING_DATA events: 0
    BUILD_ID events: 0
    FINISHED_ROUND events: 3

    ouch - no SAMPLE or TRACING_DATA events at all!

    Lets try it without the filter:

    aldebaran:~> perf record -e syscalls:sys_enter_close ./hackbench 1
    Time: 0.043
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.010 MB perf.data (~448 samples) ]
    aldebaran:~> perf report -D | tail -15
    TOTAL events: 148
    MMAP events: 59
    LOST events: 0
    COMM events: 2
    EXIT events: 3
    THROTTLE events: 0
    UNTHROTTLE events: 0
    FORK events: 40
    READ events: 0
    SAMPLE events: 41
    ATTR events: 0
    EVENT_TYPE events: 0
    TRACING_DATA events: 0
    BUILD_ID events: 0
    FINISHED_ROUND events: 3

    Got 41 SAMPLE events there.

    So the --filter stuff does not seem to work at all for syscall events.

    I tried my luck with another event: irq:irq_handler_entry. This seemed to work better:

    aldebaran:~> perf record -a -e irq:irq_handler_entry sleep 1; perf report -D | tail -15[ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.335 MB perf.data (~14630 samples) ]
    TOTAL events: 4635
    MMAP events: 2689
    LOST events: 0
    COMM events: 548
    EXIT events: 64
    THROTTLE events: 0
    UNTHROTTLE events: 0
    FORK events: 31
    READ events: 0
    SAMPLE events: 1299
    ATTR events: 0
    EVENT_TYPE events: 0
    TRACING_DATA events: 0
    BUILD_ID events: 0
    FINISHED_ROUND events: 4

    and produced a real trace:

    swapper-0 [000] 49985.641036: irq_handler_entry: irq=16 name=uhci_hcd:usb3
    swapper-0 [000] 49985.641038: irq_handler_entry: irq=16 name=eth2
    distcc-20461 [000] 49985.641088: irq_handler_entry: irq=16 name=uhci_hcd:usb3
    distcc-20461 [000] 49985.641090: irq_handler_entry: irq=16 name=eth2
    swapper-0 [000] 49985.641140: irq_handler_entry: irq=16 name=uhci_hcd:usb3
    swapper-0 [000] 49985.641141: irq_handler_entry: irq=16 name=eth2
    swapper-0 [000] 49985.641186: irq_handler_entry: irq=16 name=uhci_hcd:usb3

    So i thought i could finally test all the cool --filter stuff!

    I wanted to trace the timer irq, so i tried:

    aldebaran:~> perf record -a -e irq:irq_handler_entry --filter 'irq==0' sleep 1; perf report -D | tail -15
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.744 MB perf.data (~32519 samples) ]
    TOTAL events: 10360
    MMAP events: 6870
    LOST events: 0
    COMM events: 1205
    EXIT events: 1332
    THROTTLE events: 0
    UNTHROTTLE events: 0
    FORK events: 656
    READ events: 0
    SAMPLE events: 293
    ATTR events: 0
    EVENT_TYPE events: 0
    TRACING_DATA events: 0
    BUILD_ID events: 0
    FINISHED_ROUND events: 4

    I had to do the -D trick because otherwise I had no idea whether there were any REAL
    samples recorded...

    But 'perf script' output gave me:

    cc1-27479 [000] 50100.959394: irq_handler_entry: irq=16 name=eth2
    cc1-27479 [000] 50100.959636: irq_handler_entry: irq=16 name=uhci_hcd:usb3
    cc1-27479 [000] 50100.959638: irq_handler_entry: irq=16 name=eth2
    cc1-27479 [000] 50100.959711: irq_handler_entry: irq=0 name=timer
    cc1-27479 [000] 50100.959880: irq_handler_entry: irq=16 name=uhci_hcd:usb3
    cc1-27479 [000] 50100.959883: irq_handler_entry: irq=16 name=eth2

    Hm, filtering didnt work. Lets try to filter only that irq 16 thing then:

    aldebaran:~> perf record -a -e irq:irq_handler_entry --filter 'irq==16' sleep 1; perf report -D | tail -15
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.299 MB perf.data (~13053 samples) ]
    TOTAL events: 4118
    MMAP events: 2993
    LOST events: 0
    COMM events: 589
    EXIT events: 144
    THROTTLE events: 0
    UNTHROTTLE events: 0
    FORK events: 73
    READ events: 0
    SAMPLE events: 314
    ATTR events: 0
    EVENT_TYPE events: 0
    TRACING_DATA events: 0
    BUILD_ID events: 0
    FINISHED_ROUND events: 5

    Ok, this finally seems to have worked! I saw:

    cc1-3833 [000] 50250.934705: irq_handler_entry: irq=16 name=uhci_hcd:usb3
    cc1-3833 [000] 50250.934708: irq_handler_entry: irq=16 name=eth2
    as-3834 [000] 50250.943321: irq_handler_entry: irq=16 name=uhci_hcd:usb3
    as-3834 [000] 50250.943325: irq_handler_entry: irq=16 name=eth2
    as-3834 [000] 50250.943367: irq_handler_entry: irq=16 name=uhci_hcd:usb3

    Except that it didnt - sometimes the trace started with:

    cc1-3833 [000] 50250.916784: irq_handler_entry: irq=19 name=ahci
    cc1-3833 [000] 50250.916790: irq_handler_entry: irq=19 name=uhci_hcd:usb7
    cc1-3833 [000] 50250.917680: irq_handler_entry: irq=0 name=timer
    cc1-3833 [000] 50250.918112: irq_handler_entry: irq=19 name=ahci
    cc1-3833 [000] 50250.918119: irq_handler_entry: irq=19 name=uhci_hcd:usb7
    cc1-3833 [000] 50250.930152: irq_handler_entry: irq=16 name=uhci_hcd:usb3
    cc1-3833 [000] 50250.930156: irq_handler_entry: irq=16 name=eth2
    cc1-3833 [000] 50250.930367: irq_handler_entry: irq=16 name=uhci_hcd:usb3
    cc1-3833 [000] 50250.930369: irq_handler_entry: irq=16 name=eth2

    ouch - both irq 19 and irq 0 entries there - clearly not what i expected to see.

    Anyway, i ignored the extra lines i did not want to see and tried to use the filter
    stuff some more, to match on the 'name' field. Wanting to see both usb3 and usb7
    interrupts i typed the obvious:

    aldebaran:~> perf record -a -e irq:irq_handler_entry --filter 'name==uhci_hcd:usb' sleep 1

    But got nothing but a (not so obvious) timer irq:

    aldebaran:~> perf script
    :32749-32749 [000] 50514.340198: irq_handler_entry: irq=0 name=timer

    Ok, i thought "these guys must be doing this via wildcards, lets try that":

    aldebaran:~> perf record -a -e irq:irq_handler_entry --filter 'name==uhci_hcd:usb*'

    That didnt work either.

    So ... i have to say that this tracing filter business is unusable crap from a user
    POV right now and i see no reason to pull *anything* in this area until it does not
    get improved to *much* better levels of usability and utility.

    Nobody could *ever* have tested this with a 'naive but curious user' hat on and this
    is really sad. We need to do much better!

    Thanks,

    Ingo


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