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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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