Re: regression caused by cgroups optimization in 3.17-rc2

[Dave Hansen <dave.hansen@intel.com>]

On 09/05/2014 05:35 AM, Johannes Weiner wrote:
> On Thu, Sep 04, 2014 at 01:27:26PM -0700, Dave Hansen wrote:
>> On 09/04/2014 07:27 AM, Michal Hocko wrote:
>>> Ouch. free_pages_and_swap_cache completely kills the uncharge batching
>>> because it reduces it to PAGEVEC_SIZE batches.
>>>
>>> I think we really do not need PAGEVEC_SIZE batching anymore. We are
>>> already batching on tlb_gather layer. That one is limited so I think
>>> the below should be safe but I have to think about this some more. There
>>> is a risk of prolonged lru_lock wait times but the number of pages is
>>> limited to 10k and the heavy work is done outside of the lock. If this
>>> is really a problem then we can tear LRU part and the actual
>>> freeing/uncharging into a separate functions in this path.
>>>
>>> Could you test with this half baked patch, please? I didn't get to test
>>> it myself unfortunately.
>>
>> 3.16 settled out at about 11.5M faults/sec before the regression.  This
>> patch gets it back up to about 10.5M, which is good.  The top spinlock
>> contention in the kernel is still from the resource counter code via
>> mem_cgroup_commit_charge(), though.
> 
> Thanks for testing, that looks a lot better.
> 
> But commit doesn't touch resource counters - did you mean try_charge()
> or uncharge() by any chance?

I don't have the perf output that I was looking at when I said this, but
here's the path that I think I was referring to.  The inlining makes
this non-obvious, but this memcg_check_events() calls
mem_cgroup_update_tree() which is contending on mctz->lock.

So, you were right, it's not the resource counters code, it's a lock in
'struct mem_cgroup_tree_per_zone'.  But, the contention isn't _that_
high (2% of CPU) in this case.  But, that is 2% that we didn't see before.

>      1.87%     1.87%  [kernel]               [k] _raw_spin_lock_irqsave       
>                                |
>                                --- _raw_spin_lock_irqsave
>                                   |          
>                                   |--107.09%-- memcg_check_events
>                                   |          |          
>                                   |          |--79.98%-- mem_cgroup_commit_charge
>                                   |          |          |          
>                                   |          |          |--99.81%-- do_cow_fault
>                                   |          |          |          handle_mm_fault
>                                   |          |          |          __do_page_fault
>                                   |          |          |          do_page_fault
>                                   |          |          |          page_fault
>                                   |          |          |          testcase
>                                   |          |           --0.19%-- [...]


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