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

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