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