Re: [RFC PATCH] mm/memcontrol: Increase threshold for draining per-cpu stocked bytes.

[Imran Khan <imran.f.khan@oracle.com>]

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On 6/1/21 5:45 am, Roman Gushchin wrote:
> On Tue, Jan 05, 2021 at 10:23:52AM -0800, Roman Gushchin wrote:
>> On Tue, Jan 05, 2021 at 04:07:42PM +0000, Imran Khan wrote:
>>> While allocating objects whose size is multiple of PAGE_SIZE,
>>> say kmalloc-4K, we charge one page for extra bytes corresponding
>>> to the obj_cgroup membership pointer and remainder of the charged
>>> page gets added to per-cpu stocked bytes. If this allocation is
>>> followed by another allocation of the same size, the stocked bytes
>>> will not suffice and thus we endup charging an extra page
>>> again for membership pointer and remainder of this page gets added
>>> to per-cpu stocked bytes. This second addition will cause amount of
>>> stocked bytes to go beyond PAGE_SIZE and hence will result in
>>> invocation of drain_obj_stock.
>>>
>>> So if we are in a scenario where we are consecutively allocating,
>>> several PAGE_SIZE multiple sized objects, the stocked bytes will
>>> never be enough to suffice a request and every second request will
>>> trigger draining of stocked bytes.
>>>
>>> For example invoking __alloc_skb multiple times with
>>> 2K < packet size < 4K will give a call graph like:
>>>
>>> __alloc_skb
>>>      |
>>>      |__kmalloc_reserve.isra.61
>>>      |    |
>>>      |    |__kmalloc_node_track_caller
>>>      |    |    |
>>>      |    |    |slab_pre_alloc_hook.constprop.88
>>>      |    |     obj_cgroup_charge
>>>      |    |    |    |
>>>      |    |    |    |__memcg_kmem_charge
>>>      |    |    |    |    |
>>>      |    |    |    |    |page_counter_try_charge
>>>      |    |    |    |
>>>      |    |    |    |refill_obj_stock
>>>      |    |    |    |    |
>>>      |    |    |    |    |drain_obj_stock.isra.68
>>>      |    |    |    |    |    |
>>>      |    |    |    |    |    |__memcg_kmem_uncharge
>>>      |    |    |    |    |    |    |
>>>      |    |    |    |    |    |    |page_counter_uncharge
>>>      |    |    |    |    |    |    |    |
>>>      |    |    |    |    |    |    |    |page_counter_cancel
>>>      |    |    |
>>>      |    |    |
>>>      |    |    |__slab_alloc
>>>      |    |    |    |
>>>      |    |    |    |___slab_alloc
>>>      |    |    |    |
>>>      |    |    |slab_post_alloc_hook
>>>
>>> This frequent draining of stock bytes and resultant charging of pages
>>> increases the CPU load and hence deteriorates the scheduler latency.
>>>
>>> The above mentioned scenario and it's impact can be seen by running
>>> hackbench with large packet size on v5.8 and subsequent kernels. The
>>> deterioration in hackbench number starts appearing from v5.9 kernel,
>>> 'commit f2fe7b09a52b ("mm: memcg/slab: charge individual slab objects
>>> instead of pages")'.
>>>
>>> Increasing the draining limit to twice of KMALLOC_MAX_CACHE_SIZE
>>> (a safe upper limit for size of slab cache objects), will avoid draining
>>> of stock, every second allocation request, for the above mentioned
>>> scenario and hence will reduce the CPU load for such cases. For
>>> allocation of smaller objects or other allocation patterns the behaviour
>>> will be same as before.
>>>
>>> This change increases the draining threshold for per-cpu stocked bytes
>>> from PAGE_SIZE to KMALLOC_MAX_CACHE_SIZE * 2.
>> Hello, Imran!
>>
>> Yes, it makes total sense to me.

Hi Roman,

Thanks for reviewing this patch.

>>
>> Btw, in earlier versions of the new slab controller there was a separate stock
>> for byte-sized charging and it was 32 pages large. Later Johannes suggested
>> the current layered design and he thought that because of the layering a single
>> page is enough for the upper layer.
>>
>>> Below are the hackbench numbers with and without this change on
>>> v5.10.0-rc7.
>>>
>>> Without this change:
>>>      # hackbench process 10 1000 -s 100000
>>>      Running in process mode with 10 groups using 40 file descriptors
>>>      each (== 400 tasks)
>>>      Each sender will pass 100 messages of 100000 bytes
>>>      Time: 4.401
>>>
>>>      # hackbench process 10 1000 -s 100000
>>>      Running in process mode with 10 groups using 40 file descriptors
>>>      each (== 400 tasks)
>>>      Each sender will pass 100 messages of 100000 bytes
>>>      Time: 4.470
>>>
>>> With this change:
>>>      # hackbench process 10 1000 -s 100000
>>>      Running in process mode with 10 groups using 40 file descriptors
>>>      each (== 400 tasks)
>>>      Each sender will pass 100 messages of 100000 bytes
>>>      Time: 3.782
>>>
>>>      # hackbench process 10 1000 -s 100000
>>>      Running in process mode with 10 groups using 40 file descriptors
>>>      each (== 400 tasks)
>>>      Each sender will pass 100 messages of 100000 bytes
>>>      Time: 3.827
>>>
>>> As can be seen the change gives an improvement of about 15% in hackbench
>>> numbers.
>>> Also numbers obtained with the change are inline with those obtained
>>> from v5.8 kernel.
>> The difference is quite impressive!
>>
>> I wonder if you tried smaller values than KMALLOC_MAX_CACHE_SIZE * 2?
>> Let's say 16 and 32?

I have tested my change with smaller sizes as well and could see similar difference
in hackbench numbers

Without change(5.10.0-rc7 vanilla):

# hackbench process 10 1000 -s 16
Running in process mode with 10 groups using 40 file descriptors each (== 400 tasks)
Each sender will pass 100 messages of 16 bytes
Time: 0.429

# hackbench process 10 1000 -s 32
Running in process mode with 10 groups using 40 file descriptors each (== 400 tasks)
Each sender will pass 100 messages of 32 bytes
Time: 0.458

With my changes on top of 5.10.0-rc7
# hackbench process 10 1000 -s 16
Running in process mode with 10 groups using 40 file descriptors each (== 400 tasks)
Each sender will pass 100 messages of 16 bytes
Time: 0.347

# hackbench process 10 1000 -s 32
Running in process mode with 10 groups using 40 file descriptors each (== 400 tasks)
Each sender will pass 100 messages of 32 bytes
Time: 0.324

I am confirming using BCC based argdist tool that these sizes result in call to
__alloc_skb with size as 16 and 32 respectively.

>>
>> KMALLOC_MAX_CACHE_SIZE * 2 makes sense to me, but then the whole construction
>> with two layer caching is very questionable. Anyway, it's not a reason to not
>> merge your patch, just something I wanna look at later.
> Hm, can you, please, benchmark the following change (without your change)?
>
> @@ -3204,7 +3204,7 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
>   
>   		if (nr_pages) {
>   			rcu_read_lock();
> -			__memcg_kmem_uncharge(obj_cgroup_memcg(old), nr_pages);
> +			refill_stock(obj_cgroup_memcg(old), nr_pages);
>   			rcu_read_unlock();
>   		}

I have tested this change on top of v5.10-rc7 and this too gives performance improvement.
I further confirmed using flamegraphs that with this change too we are avoiding following
CPU intensive path

|__memcg_kmem_uncharge
     |
     |page_counter_uncharge
     |    |
     |    |page_counter_cancel

Please find the hackbench numbers with your change as given below:
# hackbench process 10 1000 -s 100000
Running in process mode with 10 groups using 40 file descriptors each (== 400 tasks)
Each sender will pass 100 messages of 100000 bytes
Time: 3.841

# hackbench process 10 1000 -s 100000
Running in process mode with 10 groups using 40 file descriptors each (== 400 tasks)
Each sender will pass 100 messages of 100000 bytes
Time: 3.863

# hackbench process 10 1000 -s 16
Running in process mode with 10 groups using 40 file descriptors each (== 400 tasks)
Each sender will pass 100 messages of 16 bytes
Time: 0.306

# hackbench process 10 1000 -s 32
Running in process mode with 10 groups using 40 file descriptors each (== 400 tasks)
Each sender will pass 100 messages of 32 bytes
Time: 0.320

Thanks,
Imran

>   

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