Most of the time you won't see any difference when using STP_ALLOC_FLAGS
in place of STP_ALLOC_SLEEP_FLAGS. You would see a difference if your
system is under memory pressure.
Let me refresh everyone's knowledge of the difference between
STP_ALLOC_FLAGS and STP_ALLOC_SLEEP_FLAGS. When using STP_ALLOC_FLAGS,
if memory isn't available, the allocation will fail. When using
STP_ALLOC_SLEEP_FLAGS, the kernel will try to wait until memory is
available. Obviously, we can only use STP_ALLOC_SLEEP_FLAGS in certain
places in the code where sleeping is permitted.
If changing STP_ALLOC_SLEEP_FLAGS to STP_ALLOC_FLAGS makes a difference,
this really means that we're using STP_ALLOC_SLEEP_FLAGS in a place
where it isn't safe to sleep.
It could be that in the normal kernel it is safe to sleep in a spot
where we're using STP_ALLOC_SLEEP_FLAGS, but in the realtime kernel it
isn't safe to sleep at that same spot. I guess it also makes sense to
sleep as little as possible when running systemtap on the realtime kernel.
So, I'll withdraw my objection. As long as the switch from
STP_ALLOC_SLEEP_FLAGS to STP_ALLOC_FLAGS only affects the realtime
kernel, I'm OK with it.