Re: [4.2, Regression] Queued spinlocks cause major XFS performance regression
From: Dave Chinner
Date: Fri Sep 04 2015 - 04:30:31 EST
On Fri, Sep 04, 2015 at 05:11:43PM +1000, Dave Chinner wrote:
> On Thu, Sep 03, 2015 at 11:39:21PM -0700, Linus Torvalds wrote:
> > There doesn't seem to be anything even remotely strange going on in that area.
> > Is this a PARAVIRT configuration? There were issues with PV
> > interaction at some point. If it is PV, and you don't actually use PV,
> > can you test with PV support disabled?
> $ grep PARAVIRT .config
> # CONFIG_PARAVIRT_DEBUG is not set
> # CONFIG_PARAVIRT_SPINLOCKS is not set
> I'll retest with CONFIG_PARAVIRT=n....
$ grep PARAVIRT .config
# CONFIG_PARAVIRT is not set
FSUse% Count Size Files/sec App Overhead
0 1600000 0 123407.7 9202289
0 3200000 0 97271.9 9187905
0 4800000 0 101010.3 11246527
So, no, that doesn't affect the queued spinlock performance at all.
> > Also, if you look at the instruction-level profile for
> > queued_spin_lock_slowpath itself, does anything stand out? For
> > example, I note that the for-loop with the atomic_cmpxchg() call in it
> > doesn't ever do a cpu_relax(). It doesn't look like that should
> > normally loop, but obviously that function also shouldn't normally use
> > 2/3rds of the cpu, so.. Maybe some part of queued_spin_lock_slowpath()
> > stands out as "it's spending 99% of the time in _that_ particular
> > part, and it gives some clue what goes wrong.
> I'll have a look when the current tests on that machine have
> finished running.
¿ Disassembly of section load2:
¿ ffffffff810e0f30 <load2+0xe0f30>:
0.00 ¿ nop
¿ push %rbp
0.00 ¿ mov %rsp,%rbp
0.00 ¿ xchg %ax,%ax
¿ xor %eax,%eax
0.00 ¿ mov $0x1,%edx
¿ lock cmpxchg %edx,(%rdi)
0.33 ¿ xor %ecx,%ecx
¿ test %eax,%eax
¿ ¿ je 28
0.02 ¿ 1c: pause
4.45 ¿ mov %ecx,%eax
0.00 ¿ lock cmpxchg %edx,(%rdi)
95.18 ¿ test %eax,%eax
¿ ¿ jne 1c
0.01 ¿ 28: pop %rbp
0.01 ¿ ¿ retq
It looks like it's spending all it's time looping around the cmpxchg.
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