Re: Btrfs for mainline

[Matthew Wilcox]

On Tue, Jan 06, 2009 at 01:47:23AM +1100, Nick Piggin wrote:
> adaptive locks have traditionally (read: Linus says) indicated the locking
> is suboptimal from a performance perspective and should be reworked. This
> is definitely the case for the -rt patchset, because they deliberately
> trade performance by change even very short held spinlocks to sleeping locks. 
> 
> So I don't really know if -rt justifies adaptive locks in mainline/btrfs.
> Is there no way for the short critical sections to be decoupled from the
> long/sleeping ones?

I wondered about that option too.  Let's see if we have other users that
will benefit from adaptive locks -- my gut says that Linus is right, but
then there's a lot of lazy programmers out there using mutexes when they
should be using spinlocks.

I wonder about a new lockdep-style debugging option that adds a bit per
mutex class to determine whether the holder ever slept while holding it.
Then a periodic check could determine which mutexes were needlessly held
would find one style of bad lock management.

The comment in btrfs certainly indicates that locking redesign is a
potential solution:

 * locks the per buffer mutex in an extent buffer.  This uses adaptive locks
 * and the spin is not tuned very extensively.  The spinning does make a big
 * difference in almost every workload, but spinning for the right amount of
 * time needs some help.
 *
 * In general, we want to spin as long as the lock holder is doing btree searches,
 * and we should give up if they are in more expensive code.

btrfs almost wants its own hybrid locks (like lock_sock(), to choose
a new in-tree example).  One where it will spin, unless a flag is set
to not spin, in which case it sleeps.  Then the 'more expensive code'
can set the flag to not bother spinning.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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