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Date: Mon, 10 May 2010 09:55:59 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Benjamin Herrenschmidt <benh@...nel.crashing.org> Cc: Frederic Weisbecker <fweisbec@...il.com>, Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...e.hu>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Tony Breeds <tonyb@....ibm.com>, Linus Torvalds <torvalds@...ux-foundation.org> Subject: Re: [PATCH/RFC] mutex: Fix optimistic spinning vs. BKL On Sat, 2010-05-08 at 08:27 +1000, Benjamin Herrenschmidt wrote: > > Also, to solve this problem when several cpus may spin on the owner, > > I wonder adaptive spinning is doing the right thing here. We should > > have only one spinner I think, and the rest should go to sleep as the > > time to spin on several subsequent owners would be much better gained > > to do something else (schedule something else or power saving). > > In fact, that too could deserve some tests. > > Right, the problem is due to the fact that we skip spinning if there's > already a waiter but we don't know that there is already a spinner so we > can end up with multiple spinners. > > I don't see a non invasive way to fix that.. we could add a spinner > counter to the mutex but that sucks a bit. Might still be worthwhile, > not sure. Peter, what do you reckon ? If its a large problem the lock is overly contended and _that_ needs fixing. I don't at all feel like adding atomic ops to the spin loop to try and detect this. 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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