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    SubjectRe: [RFC][PATCH 2/3] perf: Take a hot regs snapshot for trace events

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

    > On Thu, 2010-03-04 at 12:25 +0100, Ingo Molnar wrote:
    > > * Peter Zijlstra <peterz@infradead.org> wrote:
    > >
    > > > On Wed, 2010-03-03 at 12:07 -0500, Steven Rostedt wrote:
    > > > > oops, my bad :-), I thought this was in the x86 arch directory. For the
    > > > > University, I was helping them with adding trace points for page faults
    > > > > when I came across this in arch/x86/mm/fault.c:
    > > > >
    > > > > perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, 0, regs, address);
    > > > >
    > > > >
    > > > > This is what I actually was wondering about. Why is it a "perf only" trace
    > > > > point instead of a TRACE_EVENT()?
    > > >
    > > > Because I wanted to make perf usable without having to rely on funny
    > > > tracepoints. That is, I am less worried about committing software counters
    > > > to ABI than I am about TRACE_EVENT(), which still gives me a terribly
    > > > uncomfortable feeling.
    > >
    > > I'd still like a much less error-prone and work-intense way of doing it.
    > >
    > > I'd suggest we simply add a TRACE_EVENT_ABI() for such cases, where we
    > > really want to expose a tracepoint to tooling, programmatically. Maybe
    > > even change the usage sites to trace_foo_ABI(), to make it really clear
    > > and to make people aware of the consequences.
    >
    > Would this still be available as a normal trace event?

    Yeah, of course. It would not result in any usage or flexibility restriction.

    ( In the future we might want to add some sort of automated signature thing to
    make sure that an event that has been declared an 'ABI' isnt changed - at
    least as far as the record format goes. )

    > >
    > > > Also, building with all CONFIG_TRACE_*=n will still yield a usable perf,
    > > > which is something the embedded people might fancy, all that TRACE stuff
    > > > adds lots of code.
    > >
    > > Not a real issue i suspect when you do lock profiling ...
    > >
    > > Or if it is, some debloating might be in order - and the detaching of event
    > > enumeration and ftrace TRACE_EVENT infrastructure from other ftrace bits. (i
    > > suggested an '/eventfs' special filesystem before, for nicely layed out
    > > hierarchy of ftrace/perf events.)
    >
    > Actually, we already have a way to decouple it.
    >
    > include/trace/define_trace.h is the file that just adds the tracepoint
    > that is needed.
    >
    > include/trace/ftrace.h is the file that does the magic and adds the code
    > for callbacks and tracing.
    >
    > The perf hooks probably should not have gone in that file and been put
    > into a include/trace/perf.h file, and then in define_trace.h we would
    > add:
    >
    > #ifdef CONFIG_EVENT_TRACING
    > #include <trace/ftrace.h>
    > #endif
    >
    > +#ifdef CONFIG_PERF_EVENTS
    > +#include <trace/perf.h>
    > +#endif
    >
    > This should be done anyway. But it would also let you decouple ftrace trace
    > events from perf trace events but still let the two use the same trace
    > points.

    I think the main thing would be to have a decoupled /eventfs - basically
    /debug/tracing/events/ moved to "/eventfs" or maybe to "/proc/events/". This
    would make them available more widely, and in a standardized way.

    Thanks,

    Ingo


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