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Re: [PATCH 05/05] Linux Kernel Markers, non optimised architectures
- From: Mathieu Desnoyers <mathieu dot desnoyers at polymtl dot ca>
- To: Nick Piggin <nickpiggin at yahoo dot com dot au>
- Cc: linux-kernel at vger dot kernel dot org, Linus Torvalds <torvalds at osdl dot org>, Andrew Morton <akpm at osdl dot org>, Ingo Molnar <mingo at redhat dot com>, Greg Kroah-Hartman <gregkh at suse dot de>, Christoph Hellwig <hch at infradead dot org>, ltt-dev at shafik dot org, systemtap at sources dot redhat dot com, Douglas Niehaus <niehaus at eecs dot ku dot edu>, Thomas Gleixner <tglx at linutronix dot de>
- Date: Fri, 12 Jan 2007 00:00:32 -0500
- Subject: Re: [PATCH 05/05] Linux Kernel Markers, non optimised architectures
- References: <11685601382063-git-send-email-mathieu.desnoyers@polymtl.ca> <11685601404005-git-send-email-mathieu.desnoyers@polymtl.ca> <45A710F8.7000405@yahoo.com.au>
* Nick Piggin (nickpiggin@yahoo.com.au) wrote:
> Mathieu Desnoyers wrote:
>
> >+#define MARK(name, format, args...) \
> >+ do { \
> >+ static marker_probe_func *__mark_call_##name = \
> >+ __mark_empty_function; \
> >+ volatile static char __marker_enable_##name = 0; \
> >+ static const struct __mark_marker_c __mark_c_##name \
> >+ __attribute__((section(".markers.c"))) = \
> >+ { #name, &__mark_call_##name, format } ; \
> >+ static const struct __mark_marker __mark_##name \
> >+ __attribute__((section(".markers"))) = \
> >+ { &__mark_c_##name, &__marker_enable_##name } ; \
> >+ asm volatile ( "" : : "i" (&__mark_##name)); \
> >+ __mark_check_format(format, ## args); \
> >+ if (unlikely(__marker_enable_##name)) { \
> >+ preempt_disable(); \
> >+ (*__mark_call_##name)(format, ## args); \
> >+ preempt_enable_no_resched(); \
>
> Why not just preempt_enable() here?
>
Because the preempt_enable() macro contains preempt_check_resched(), which
may call preempt_schedule() which leads us to a call to schedule(). Therefore,
all those very interesting scheduler functions would cause an infinite
recursive scheduler call if we marked schedule() and used preempt_enable() in
the marker.
The primary goal for the markers (and the probes that attaches to them) is to
have the fewest side-effects possible : any kernel method called from an
instrumentation site adds this precise kernel method to the "cannot be
instrumented" list, which I want to keep as small possible.
Mathieu
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