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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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