[Bug runtime/11592] New: sdt.h should have a 'disable-semaphores' override option

[Bug runtime/11592] New: sdt.h should have a 'disable-semaphores' override option

[fche at redhat dot com]

See https://bugzilla.gnome.org/show_bug.cgi?id=606044
for a situation where semaphores constitute a performance
regression.  (In this case, argument setup is small compared
to the semaphore value check.)

Unless we can automate per-marker semaphore presence, we should
provide an override capability.  It may need to be nothing larger
than allowing an application .c file to define

#define STAP_NO_SEMAPHORES

which then the dtrace.in-generated script would observe and avoid
defining STAP_HAS_SEMAPHORES.

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           Summary: sdt.h should have a 'disable-semaphores' override option
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: runtime
        AssignedTo: systemtap at sources dot redhat dot com
        ReportedBy: fche at redhat dot com
                CC: scox at redhat dot com


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[Bug runtime/11592] sdt.h should have a 'disable-semaphores' override option

[alexl at redhat dot com]

------- Additional Comments From alexl at redhat dot com  2010-05-12 15:20 -------
Why doesn't semaphores work like the is-enabled stuff in dtrace:
http://blogs.sun.com/ahl/entry/user_land_tracing_gets_better

Seems like that would be much more useful than automatically enabling it for all
probes. And its dtrace compatible to boot.


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           What    |Removed                     |Added
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                 CC|                            |alexl at redhat dot com


http://sourceware.org/bugzilla/show_bug.cgi?id=11592

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[Bug runtime/11592] sdt.h should have a 'disable-semaphores' override option

[fche at redhat dot com]

------- Additional Comments From fche at redhat dot com  2010-05-12 15:24 -------
(In reply to comment #1)
> Why doesn't semaphores work like the is-enabled stuff in dtrace:

Right, that foo_IS_ENABLED() is already supported as is.

> Seems like that would be much more useful than automatically enabling it for all
> probes.

The argument has been that we've encountered cases where sdt.h macros
were called with fairly heavy-weight arguments, but without an
explicit foo_IS_ENABLED() wrapper.  In these cases, it helped.  But now
there is evidence that it also hurts sometimes, so we need to re-evaluate.



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[Bug runtime/11592] sdt.h should have a 'disable-semaphores' override option

[alexl at redhat dot com]

------- Additional Comments From alexl at redhat dot com  2010-05-14 07:20 -------
I don't particularly care if semaphores are automatically enabled for probes or
not, as long as there is a way to enable/disable it. For glib i want to disable
it and do it manually where needed. This should imho be the right approach for
all probe sets, but I realize not everyone need to be as careful about
performance issues as core glib functionallity.

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[Bug runtime/11592] sdt.h should have a 'disable-semaphores' override option

[walters at verbum dot org]

------- Additional Comments From walters at verbum dot org  2010-05-25 13:30 -------
Created an attachment (id=4811)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=4811&action=view)
0001-dtrace.in-Allow-definition-of-STAP_LIGHTWEIGHT_PROBE.patch

Something like this (untested) patch?

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[Bug runtime/11592] sdt.h should have a 'disable-semaphores' override option

[fche at redhat dot com]

------- Additional Comments From fche at redhat dot com  2010-05-25 13:35 -------
Or maybe we should flip the default over to no-semaphores, and have
a user specifically request it:

-D STAP_SDT_IMPLICIT_SEMAPHORES

(since they can already use foo_ENABLED() for explicit conditions).


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[Bug runtime/11592] sdt.h should have a 'disable-semaphores' override option

[alexl at redhat dot com]

------- Additional Comments From alexl at redhat dot com  2010-05-25 15:11 -------
I agree with frank. Additionally, thats what dtrace does.

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[Bug runtime/11592] sdt.h should have a 'disable-semaphores' override option

[scox at redhat dot com]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|systemtap at sources dot    |scox at redhat dot com
                   |redhat dot com              |
             Status|NEW                         |ASSIGNED


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