Re: [4.2, Regression] Queued spinlocks cause major XFS performance regression

From: Linus Torvalds
Date: Fri Sep 04 2015 - 11:05:25 EST

On Fri, Sep 4, 2015 at 1:29 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> 0.02 1c: pause
> 4.45 mov %ecx,%eax
> 0.00 lock cmpxchg %edx,(%rdi)
> 95.18 test %eax,%eax
> jne 1c
> It looks like it's spending all it's time looping around the cmpxchg.

That code sequence doesn't look sensible. Busy-looping on a cmpxchg is
insane - if you are busy-looping, you should always make sure the
inner tight loop is done while waiting for the value.

It seems to come from virt_queued_spin_lock(), and that just looks
like completely bogus crap.

PeterZ, this is your magical hypervisor thing, and I get the feeling
that that explains why Dave sees nasty performance: most people have
tested either on raw hardware or using the actual paravirtualized
ones, but this is the case for "we're running with a hypervisor, but
not paravirtualized".

So virt_queued_spin_lock() for the hypervisor case looks completely
buggered to me for several reasons:

- it doesn't actually ever use any queueing, since it always returns true

so the "queued spinlocks" in this case aren't actually queued, and
they aren't even ticket-locks, they are just plain 0/1 values if I
read things right.

- the busy-loop to set the queued spinlock uses that cmpxchg in a
tight loop, which kills any memory subsystem. That's unacceptable.

So at the very *minimum*, that second issue should be fixed, and the
loop in virt_queued_spin_lock() should look something like

do {
while (READ_ONCE(lock->val) != 0)
} while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);

which at least has a chance in hell of behaving well on the bus and in
a HT environment.

But I suspect that it would be even better for Dave to just disable
the whole thing, and see how the queued locks actually work. Dave, can
you turn that virt_queued_spin_lock() into just "return false"? In
fact, I would almost _insist_ we do this when CONFIG_PARAVIRT_SPINLOCK
isn't set, isn't that what our old ticket-spinlocks did? They didn't
screw up and degrade to a test-and-set lock just because they saw a
hypervisor - that only happened when things were paravirt-aware. No?

Dave, if you have the energy, try it both ways. But the code as-is for
"I'm running in a hypervisor" looks just terminally broken. People who
didn't run in hypervisors just never saw the breakage.

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