Messages in this thread | | | Subject | Re: [PATCH/RFC] mutex: Fix optimistic spinning vs. BKL | From | Peter Zijlstra <> | Date | Tue, 11 May 2010 20:19:40 +0200 |
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On Tue, 2010-05-11 at 11:06 -0700, Linus Torvalds wrote: > > 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.
There is a second need_resched() in the inner spin loop in kernel/sched.c:mutex_spin_on_owner().
> 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.
Granted.
> 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".
Right, so basically count the number of mutex_spin_on_owner() calls and bail when >N.
> 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.
Ben's argument was that spinning for a long time wrecks power usage.
That said, I'd still like a counter/event/warning to see if someone actually manages to hold onto a mutex for long (2 jiffies) without scheduling at all. If we ever run into something like that, that needs to get fixed regardless.
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