Re: [PATCH-next v5 3/4] mm/memcg: Improve refill_obj_stock() performance

[Waiman Long <llong@redhat.com>]

On 4/21/21 7:55 PM, Roman Gushchin wrote:
> On Tue, Apr 20, 2021 at 03:29:06PM -0400, Waiman Long wrote:
>> There are two issues with the current refill_obj_stock() code. First of
>> all, when nr_bytes reaches over PAGE_SIZE, it calls drain_obj_stock() to
>> atomically flush out remaining bytes to obj_cgroup, clear cached_objcg
>> and do a obj_cgroup_put(). It is likely that the same obj_cgroup will
>> be used again which leads to another call to drain_obj_stock() and
>> obj_cgroup_get() as well as atomically retrieve the available byte from
>> obj_cgroup. That is costly. Instead, we should just uncharge the excess
>> pages, reduce the stock bytes and be done with it. The drain_obj_stock()
>> function should only be called when obj_cgroup changes.
> I really like this idea! Thanks!
>
> However, I wonder if it can implemented simpler by splitting drain_obj_stock()
> into two functions:
>       empty_obj_stock() will flush cached bytes, but not reset the objcg
>       drain_obj_stock() will call empty_obj_stock() and then reset objcg
>
> Then we simple can replace the second drain_obj_stock() in
> refill_obj_stock() with empty_obj_stock(). What do you think?

Actually the problem is the flushing cached bytes to 
objcg->nr_charged_bytes that can become a performance bottleneck in a 
multithreaded testing scenario. See my description in the latter half of 
my cover-letter.

For cgroup v2, update the page charge will mostly update the per-cpu 
page charge stock. Flushing the remaining byte charge, however, will 
cause the obgcg to became the single contended cacheline for all the 
cpus that need to flush the byte charge. That is why I only update the 
page charge and left the remaining byte charge stayed put in the object 
stock.

>
>> Secondly, when charging an object of size not less than a page in
>> obj_cgroup_charge(), it is possible that the remaining bytes to be
>> refilled to the stock will overflow a page and cause refill_obj_stock()
>> to uncharge 1 page. To avoid the additional uncharge in this case,
>> a new overfill flag is added to refill_obj_stock() which will be set
>> when called from obj_cgroup_charge().
>>
>> A multithreaded kmalloc+kfree microbenchmark on a 2-socket 48-core
>> 96-thread x86-64 system with 96 testing threads were run.  Before this
>> patch, the total number of kilo kmalloc+kfree operations done for a 4k
>> large object by all the testing threads per second were 4,304 kops/s
>> (cgroup v1) and 8,478 kops/s (cgroup v2). After applying this patch, the
>> number were 4,731 (cgroup v1) and 418,142 (cgroup v2) respectively. This
>> represents a performance improvement of 1.10X (cgroup v1) and 49.3X
>> (cgroup v2).
> This part looks more controversial. Basically if there are N consequent
> allocations of size (PAGE_SIZE + x), the stock will end up with (N * x)
> cached bytes, right? It's not the end of the world, but do we really
> need it given that uncharging a page is also cached?

Actually the maximum charge that can be accumulated in (2*PAGE_SIZE + x 
- 1) since a following consume_obj_stock() will use those bytes once the 
byte charge is not less than (PAGE_SIZE + x).

Yes, the page charge is cached for v2, but it is not the case for v1. 
See the benchmark data in the cover-letter.

Cheers,
Longman