Re: [PATCH] mmap_lock: add tracepoints around lock acquisition

[Axel Rasmussen <axelrasmussen@google.com>]

On Thu, Sep 17, 2020 at 12:43 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Thu, 17 Sep 2020 11:13:47 -0700
> Axel Rasmussen <axelrasmussen@google.com> wrote:
>
> > +/*
> > + * Trace calls must be in a separate file, as otherwise there's a circuclar
> > + * dependency between linux/mmap_lock.h and trace/events/mmap_lock.h.
> > + */
> > +
> > +static void trace_start_locking(struct mm_struct *mm, bool write)
>
> Please don't use "trace_" for functions, as that should be reserved for the
> actual tracepoint functions. Please use "do_trace_" or whatever so there's
> no confusion about this being a tracepoint, even if it's just a function
> that calls the tracepoint.

Done; I'll send a v2 with this change.

>
> > +{
> > +     TRACE_MMAP_LOCK_EVENT(start_locking, mm, 0, write, true);
> > +}
> > +
> > +static void trace_acquire_returned(struct mm_struct *mm, u64 start_time_ns,
> > +                                bool write, bool success)
> > +{
> > +     TRACE_MMAP_LOCK_EVENT(acquire_returned, mm,
> > +                           sched_clock() - start_time_ns, write, success);
> > +}
> > +
> > +static void trace_released(struct mm_struct *mm, bool write)
> > +{
> > +     TRACE_MMAP_LOCK_EVENT(released, mm, 0, write, true);
> > +}
> > +
>
> > +static inline void lock_impl(struct mm_struct *mm,
> > +                          void (*lock)(struct rw_semaphore *), bool write)
> > +{
> > +     u64 start_time_ns;
> > +
> > +     trace_start_locking(mm, write);
> > +     start_time_ns = sched_clock();
> > +     lock(&mm->mmap_lock);
> > +     trace_acquire_returned(mm, start_time_ns, write, true);
> > +}
> > +
>
> Why record the start time and pass it in for return, when this can be done
> by simply recording the start and return and then using the timestamps of
> the trace events to calculate the duration, offline or as synthetic events:

First, thanks for the detailed feedback! As a newbie this is very helpful. :)

I agree in principle, and I almost have a working version as you
suggest, but I can't see a way to get string fields working.

I believe in trace event headers the typical way to define a string
field  is as a "const char *", with the __string, __assign_str, and
__get_str helpers. But, from reading trace_events_synth.c, this isn't
really supported, in that it only supports "char []". But, the hist
trigger code just does a strcmp() of the type string, it doesn't do
any type conversion, so it considers these types incompatible:

After this:
# echo 'mmap_lock_latency u64 time; char memcg_path[256]' >
/sys/kernel/tracing/synthetic_events

Trying to setup the hist trigger gives (the ^ points to the beginning
of keys=>m<emcg_path ... not sure the formatting will be preserved
properly in e-mail):
# cat /sys/kernel/tracing/error_log
[   15.823725] hist:mmap_lock:mmap_lock_acquire_returned: error: Param
type doesn't match synthetic event field type
  Command: hist:keys=memcg_path:latency=common_timestamp.usecs-$ts0:onmatch(mmap_lock.mmap_lock_start_locking).mmap_lock_latency($latency,memcg_path)
                     ^

I tried grepping "char [^\[]+\[" in include/trace/events/, and it
seems nobody is defining fixed-length string fields like that, so I
think that's the wrong solution. I checked the docs about defining
variables (https://www.kernel.org/doc/html/v5.2/trace/histogram.html)
and it doesn't support anything complex like a cast, just - and +.

Any advice?



>
>
>  # cd /sys/kernel/tracing/
>  # echo 'duration u64 time' > synthetic_events
>  # echo 'hist:keys=common_pid:ts0=common_timestamp.usecs" > events/mmap_lock/mmap_lock_start_locking/trigger
>  # echo 'hist:keys=common_pid:dur=common_timestamp.usecs-$ts0:onmatch(mmap_lock.mmap_lock_start_locking).trace(duration,$dur)" > events/mmap_lock/mmap_lock_acquire_returned/trigger
>  # echo 1 > events/synthetic/duration/enable
>  # cat trace
> # tracer: nop
> #
> # entries-in-buffer/entries-written: 148/148   #P:8
> #
> #                              _-----=> irqs-off
> #                             / _----=> need-resched
> #                            | / _---=> hardirq/softirq
> #                            || / _--=> preempt-depth
> #                            ||| /     delay
> #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
> #              | |       |   ||||       |         |
>             bash-1613  [007] ...3  3186.431687: duration: time=3
>             bash-1613  [007] ...3  3186.431722: duration: time=2
>             bash-1613  [007] ...3  3186.431772: duration: time=2
>             bash-1613  [001] ...3  3188.372001: duration: time=6
>             bash-1613  [001] ...3  3188.372324: duration: time=6
>             bash-1613  [001] ...3  3188.372332: duration: time=4
>             bash-1613  [001] ...3  3188.373557: duration: time=5
>             bash-1613  [001] ...3  3188.373595: duration: time=3
>              cat-1868  [002] ...3  3188.373608: duration: time=8
>             bash-1613  [001] ...3  3188.373613: duration: time=4
>             bash-1613  [001] ...3  3188.373635: duration: time=3
>              cat-1868  [002] ...3  3188.373646: duration: time=4
>             bash-1613  [001] ...3  3188.373652: duration: time=3
>             bash-1613  [001] ...3  3188.373669: duration: time=3
>
>  # echo 'hist:keys=time' > events/synthetic/duration/trigger
>  # cat events/synthetic/duration/hist
> # event histogram
> #
> # trigger info: hist:keys=time:vals=hitcount:sort=hitcount:size=2048 [active]
> #
>
> { time:        114 } hitcount:          1
> { time:         15 } hitcount:          1
> { time:         11 } hitcount:          1
> { time:         21 } hitcount:          1
> { time:         10 } hitcount:          1
> { time:         46 } hitcount:          1
> { time:         29 } hitcount:          1
> { time:         13 } hitcount:          2
> { time:         16 } hitcount:          3
> { time:          9 } hitcount:          3
> { time:          8 } hitcount:          3
> { time:          7 } hitcount:          8
> { time:          6 } hitcount:         10
> { time:          5 } hitcount:         28
> { time:          4 } hitcount:        121
> { time:          1 } hitcount:        523
> { time:          3 } hitcount:        581
> { time:          2 } hitcount:        882
>
> Totals:
>     Hits: 2171
>     Entries: 18
>     Dropped: 0
>
> And with this I could do a bunch of things like stack trace on max hits and
> other features that the tracing histograms give us.
>
> -- Steve