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SubjectRe: [PATCH/RFC] mutex: Fix optimistic spinning vs. BKL

On Mon, 10 May 2010, Peter Zijlstra wrote:
> As to the 2 jiffy spin timeout, I guess we should add a lockdep warning
> for that, because anybody holding a mutex for longer than 2 jiffies and
> not sleeping does need fixing anyway.

I really hate the jiffies thing, but looking at the optimistic spinning, I
do wonder about two things..

First - we check "need_resched()" only if owner is NULL. That sounds
wrong. If we need to reschedule, we need to stop spinning _regardless_ of
whether the owner may have been preempted before setting the owner field.

Second: we allow "owner" to change, and we'll continue spinning. This is
how you can end up spinning for a long time - not because anybody holds
the mutex for longer than 2 jiffies, but because a lot of other threads
_together_ hold the mutex for longer than 2 jiffies.

Now, I think we do want some limited "continue spinning even if somebody
else ended up getting it instead", but I think we should at least limit
it. Otherwise we end up being potentially rather unfair, since we don't
have any fair queueing logic for the optimistic spinning phase.

Now, we could just count the number of times "owner" has changed, and I
suspect that would be sufficient. Now, that trivial counting sceme would
fail if "owner" stays the same (ie the same process re-takes the lock over
and over again, possibly due to hot cacheline things being very unfair
to the person who already owns it), but quite frankly, I don't think we
can get into that kind of situation.

Why? Mutexes may end up being very heavily contended, but they can't be
contended by just _one_ thread. So if we're really in a starvation issue,
the thread that is waiting _will_ see multiple different owners.

So once you have seen X number of other owners, you just say "screw it,
this spinning thing isn't working for me, I'll go to the sleeping case".

Of course, that is _really_ unfair. It will make it all that easier for
the other spinners to just get the lock by spinning. We used to have some
logic to say "we won't spin if there are proper waiters", but that was
horrible for performance. See commit ac6e60ee405.

Of course, it's quite possible that as long as "need_resched()" isn't set,
spinning really _is_ the right thing to do. Maybe it causes horrible CPU
load on some odd "everybody synchronize" loads, but maybe that really is
the best we can do.


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