linux-kernel - Re: [PATCH 05/05] Linux Kernel Markers, non optimised architectures

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Date:	Fri, 12 Jan 2007 00:00:32 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	linux-kernel@...r.kernel.org, Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>, Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Christoph Hellwig <hch@...radead.org>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com,
	Douglas Niehaus <niehaus@...s.ku.edu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 05/05] Linux Kernel Markers, non optimised architectures

* Nick Piggin (nickpiggin@...oo.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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