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

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 18 Sep 2020 13:26:37 -0700
Axel Rasmussen <axelrasmussen@google.com> wrote:

> 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?

Tom,

Do you think we could make histograms support the above somehow?

-- Steve

> 
> 
> 
> >
> >
> >  # 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