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SubjectRe: [PATCH] mmap_lock: add tracepoints around lock acquisition
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Hi Steve, Axel,

On Fri, 2020-09-18 at 16:41 -0400, Steven Rostedt wrote:
> 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?
>

Sorry for the delayed reply - was out on vacation.

Yeah, currently the synthetic events only support constant-length
strings, which should match with __array()s in the tracepoints, but I
think they should also be made to support variable-length arrays that
would match __string() etc.

I'm thinking an array field without length specifier could be used to
in the synthetic event specification for that e.g.:

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

I'll work on adding that over the next couple days or so...

Tom

> -- 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-
> > > $ts0:onmatch(mmap_lock.mmap_lock_start_locking).trace(duration,$d
> > > ur)" > 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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