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

[Nick Piggin <nickpiggin@yahoo.com.au>]

Mathieu Desnoyers wrote:
> * 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 vast majority of schedule() has preempt turned off, so that shouldn't
be a problem, if you provide a comment.

> 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.

OK, well one problem is that it can cause a resched event to be lost, so
you might say it has more side-effects without checking resched.

-- 
SUSE Labs, Novell Inc.
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