linux-kernel - Re: Tasks stuck in futex code (in 3.14-rc6)

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Date:	Thu, 20 Mar 2014 12:08:30 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: Tasks stuck in futex code (in 3.14-rc6)

On Thu, 2014-03-20 at 11:36 -0700, Linus Torvalds wrote:
> On Thu, Mar 20, 2014 at 10:18 AM, Davidlohr Bueso <davidlohr@...com> wrote:
> >
> > Comparing with the patch I sent earlier this morning, looks equivalent,
> > and fwiw, passes my initial qemu bootup, which is the first way of
> > detecting anything stupid going on.
> >
> > So, Srikar, please try this patch out, as opposed to mine, you don't
> > have to first revert the commit in question.
> 
> Ok, so it boots for me too, so hopefully it isn't totally broken.
> 
> However, since it's just closing a race, and since getting the counts
> wrong should easily result in it *working* but always taking the slow
> path (for example), I'd really like people to also verify that it
> fixes the actual performance issue (ie assuming it fixes powerpc
> behavior for Srikar, I'd like to get it double-checked that it also
> avoids the spinlock in the common case). 

Oh, it does. This atomics technique was tested at a customer's site and
ready for upstream. To refresh, we were originally seeing massive
contention on the hb->lock and an enormous amounts of 0 returns from
futex_wake, indicating that spinners where piling up just to realize
that the plist was empty! While I don't have any official numbers, I can
confirm that perf showed that this issue was addressed with the atomics
variant. Yes, such pathological behavior shows problems in the userspace
locking primitives design/implementation, but allowing the kernel not to
be affected by suboptimal uses of futexes is definitely a plus. 

As tglx suggested at the time, I also made sure that adding the barriers
when doing the key refcounting didn't impose any serious restrictions to
performance either.

Now, what at the time required re-testing everything was when you
suggested replacing this approach with a more elegant spin is locked
test. Both approaches showed pretty much identical performance (and
correctness, at least on x86). And to this day shows *significant* less
time spent in kernel space dealing with futexes.

> Because if the
> increment/decrement pairings end up being wrong, we could have a
> situation where the waiter count just ends up bogus, and it all works
> from a correctness standpoint but not from the intended performance
> optimization.
> 
> No way I can test that sanely on my single-socket machine. Davidlohr?

Not this patch, no :( -- we could never blindly reproduce the customer's
workload. The only patch that I was able to create test cases for is the
larger hash table one, which simply alleviates collisions. This is now
part of perf-bench.

Thanks,
Davidlohr

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