Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression

Dave Chinner
2016-08-15T22:43:21Z

On Tue, Aug 16, 2016 at 08:22:11AM +1000, Dave Chinner wrote:
> On Sun, Aug 14, 2016 at 10:12:20PM -0700, Linus Torvalds wrote:
> > On Aug 14, 2016 10:00 PM, "Dave Chinner" <david@fromorbit.com> wrote:
> > >
> > > > What does it say if you annotate that _raw_spin_unlock_irqrestore()
> > function?
> > > ....
> > > ¿
> > > ¿ Disassembly of section load0:
> > > ¿
> > > ¿ ffffffff81e628b0 <load0>:
> > > ¿ nop
> > > ¿ push %rbp
> > > ¿ mov %rsp,%rbp
> > > ¿ movb $0x0,(%rdi)
> > > ¿ nop
> > > ¿ mov %rsi,%rdi
> > > ¿ push %rdi
> > > ¿ popfq
> > > 99.35 ¿ nop
> >
> > Yeah, that's a good disassembly of a non-debug spin unlock, and the symbols
> > are fine, but the profile is not valid. That's an interrupt point, right
> > after the popf that enables interiors again.
> >
> > I don't know why 'perf' isn't working on your machine, but it clearly
> > isn't.
> >
> > Has it ever worked on that machine?
>
> It's working the same as it's worked since I started using it many
> years ago.
>
> > What cpu is it?
>
> Intel(R) Xeon(R) CPU E5-4620 0 @ 2.20GHz
>
> > Are you running in some
> > virtualized environment without performance counters, perhaps?
>
> I've mentioned a couple of times in this thread that I'm testing
> inside a VM. It's the same VM I've been running performance tests in
> since early 2010. Nobody has complained that the profiles I've
> posted are useless before, and not once in all that time have they
> been wrong in indicating a spinning lock contention point.
>
> i.e. In previous cases where I've measured double digit CPU usage
> numbers in a spin_unlock variant, it's always been a result of
> spinlock contention. And fixing the algorithmic problem that lead to
> the spinlock showing up in the profile in the first place has always
> substantially improved performance and scalability.
>
> As such, I'm always going to treat a locking profile like that as
> contention because even if it isn't contending *on my machine*,
> that amount of work being done under a spinning lock is /way too
> much/ and it *will* cause contention problems with larger machines.

And, so, after helpfully being pointed at the magic kvm "-cpu host"
flag to enable access to the performance counters from the guest
(using "-e cycles", because more precise counters aren't available),
the profile looks like this:

31.18% [kernel] [k] __pv_queued_spin_lock_slowpath
9.90% [kernel] [k] copy_user_generic_string
3.65% [kernel] [k] __raw_callee_save___pv_queued_spin_unlock
2.62% [kernel] [k] __block_commit_write.isra.29
2.26% [kernel] [k] _raw_spin_lock_irqsave
1.72% [kernel] [k] _raw_spin_lock
1.33% [kernel] [k] __wake_up_bit
1.20% [kernel] [k] __radix_tree_lookup
1.19% [kernel] [k] __remove_mapping
1.12% [kernel] [k] __delete_from_page_cache
0.97% [kernel] [k] xfs_do_writepage
0.91% [kernel] [k] get_page_from_freelist
0.90% [kernel] [k] up_write
0.88% [kernel] [k] clear_page_dirty_for_io
0.83% [kernel] [k] radix_tree_tag_set
0.81% [kernel] [k] radix_tree_tag_clear
0.80% [kernel] [k] down_write
0.78% [kernel] [k] _raw_spin_unlock_irqrestore
0.77% [kernel] [k] shrink_page_list
0.76% [kernel] [k] ___might_sleep
0.76% [kernel] [k] unlock_page
0.74% [kernel] [k] __list_del_entry
0.67% [kernel] [k] __add_to_page_cache_locked
0.65% [kernel] [k] node_dirty_ok
0.61% [kernel] [k] __rmqueue
0.61% [kernel] [k] __block_write_begin_int
0.61% [kernel] [k] cancel_dirty_page
0.61% [kernel] [k] __test_set_page_writeback
0.59% [kernel] [k] page_mapping
0.57% [kernel] [k] __list_add
0.56% [kernel] [k] free_pcppages_bulk
0.54% [kernel] [k] _raw_spin_lock_irq
0.54% [kernel] [k] generic_write_end
0.51% [kernel] [k] drop_buffers

The call graph should be familiar by now:

36.60% 0.00% [kernel] [k] kswapd
- 30.29% kswapd
- 30.23% shrink_node
- 30.07% shrink_node_memcg.isra.75
- 30.15% shrink_inactive_list
- 29.49% shrink_page_list
- 22.79% __remove_mapping
- 22.27% _raw_spin_lock_irqsave
__pv_queued_spin_lock_slowpath
+ 1.86% __delete_from_page_cache
+ 1.27% _raw_spin_unlock_irqrestore
+ 4.31% try_to_release_page
+ 1.21% free_hot_cold_page_list
0.56% page_evictable
0.77% isolate_lru_pages.isra.72

That sure looks like spin lock contention to me....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com