On Sat, 12 May 2007, Ingo Molnar wrote:
* Esben Nielsen <[email protected]> wrote:
I notice that the rwsems used now isn't priority inversion safe (thus
destroying the perpose of having PI futexes). We thus already have a
bug in the mainline.
you see everything in black and white, ignoring all the grey scales!
Upstream PI futexes are perfectly fine as long as the mm semaphore is
not write-locked (by anyone) while the critical path is running. Given
that real-time tasks often use mlockall and other practices to simplify
their workload so this is not all that hard to achieve.
Yeah, after sending that mail I realized I accepted this fact way back...
But I disagree in that it is easy to avoid not write-lcling the mm
semaphore: A simple malloc() might lead to a mmap() call creating trouble.
Am I right?
I suggest making a rw_mutex which does read side PI: A reader boosts
the writer, but a writer can't boost the readers, since there can be a
large amount of those.
this happens automatically when you use Peter's stuff on -rt.
Because the rw_mutex is translated into a rt_mutex - as you say below.
But
mainline should not be bothered with this.
I disagree. You lay a large burdon on the users of PI futexes to avoid
write locking the mm semaphore. PI boosting those writers would be a good
idea even in the mainline.
I don't have time to make such a rw_mutex but I have a simple idea for
one, where the rt_mutex can be reused.
Peter's stuff does this already if you remap all the mutex ops to
rt_mutex ops. Which is also what happens on -rt automatically. Ok?
That is how -rt works and should work. No disagrement there.
[ for mainline it is totally pointless and unnecessary to slow down all
MM ops via an rt_mutex, because there are so many other, much larger
effects that make execution time unbound. (interrupts for example) ]
1) How much slower would the pi_rw_mutex I suggested really be? As far as
I see there is only an overhead when there is congestion. I can not see
that that overhead is much larger than a non-PI boosting implementation.
2) I know that execution time isn't bounded in the main-line - that is why
-rt is needed. But it is _that_ bad? How low can you get your latencies
with preemption on on a really busy machine?
Basically, I think PI futexes should provide the same kind of latencies
as the basic thread latency. That is what the user would expect.
Priority invertions should be removed, but ofcourse you can't do better
than the OS does otherwise.
Ingo
Esben
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