Re: [PATCH v3 0/3] Use kmem_cache for memcg alloc

[Huan Yang <link@vivo.com>]

Hi Matthew,

在 2025/4/25 12:35, Matthew Wilcox 写道:
> On Fri, Apr 25, 2025 at 11:19:22AM +0800, Huan Yang wrote:
>> Key Observations:
>>    1. Both structures use kmalloc with requested sizes between 2KB-4KB
>>    2. Allocation alignment forces 4KB slab usage due to pre-defined sizes
>>       (64B, 128B,..., 2KB, 4KB, 8KB)
>>    3. Memory waste per memcg instance:
>>        Base struct: 4096 - 2312 = 1784 bytes
>>        Per-node struct: 4096 - 2896 = 1200 bytes
>>        Total waste: 2984 bytes (1-node system)
>>        NUMA scaling: (1200 + 8) * nr_node_ids bytes
>> So, it's a little waste.
> [...]
>
>> This indicates that the `mem_cgroup` struct now requests 2312 bytes
>> and is allocated 2368 bytes, while `mem_cgroup_per_node` requests 2896 bytes
>> and is allocated 2944 bytes.
>> The slight increase in allocated size is due to `SLAB_HWCACHE_ALIGN` in the
>> `kmem_cache`.
>>
>> Without `SLAB_HWCACHE_ALIGN`, the allocation might appear as:
>>
>>    # mem_cgroup struct allocation
>>    sh-9269     [003] .....    80.396366: kmem_cache_alloc:
>>      call_site=mem_cgroup_css_alloc+0xbc/0x5d4 ptr=000000005b12b475
>>      bytes_req=2312 bytes_alloc=2312 gfp_flags=GFP_KERNEL|__GFP_ZERO node=-1
>>      accounted=false
>>
>>    # mem_cgroup_per_node allocation
>>    sh-9269     [003] .....    80.396411: kmem_cache_alloc:
>>      call_site=mem_cgroup_css_alloc+0x1b8/0x5d4 ptr=00000000f347adc6
>>      bytes_req=2896 bytes_alloc=2896 gfp_flags=GFP_KERNEL|__GFP_ZERO node=0
>>      accounted=false
>>
>> While the `bytes_alloc` now matches the `bytes_req`, this patchset defaults
>> to using `SLAB_HWCACHE_ALIGN` as it is generally considered more beneficial
>> for performance. Please let me know if there are any issues or if I've
>> misunderstood anything.
> This isn't really the right way to think about this.  Memory is ultimately
> allocated from the page allocator.  So what you want to know is how many
> objects you get per page.  Before, it's one per page (since both objects
> are between 2k and 4k and rounded up to 4k).  After, slab will create
> slabs of a certain order to minimise waste, but also not inflate the
> allocation order too high.  Let's assume it goes all the way to order 3
> (like kmalloc-4k does), so you want to know how many objects fit in a
> 32KiB allocation.
>
> With HWCACHE_ALIGN, you get floor(32768/2368) = 13 and
> floor(32768/2944) = 11.
>
> Without HWCACHE_ALIGN( you get floor(32768/2312) = 14 and
> floor(32768/2896) = 11.

Yes, thanks. And, this can easily observe with the following command:

   # show mem_cgroup slab's order, it's 3.
   cat /sys/kernel/slab/mem_cgroup/order
   # show mem_cgroup slab's objs per slab, it's 13
   cat /sys/kernel/slab/mem_cgroup/objs_per_slab

And we can quickly calculate the Page order obtained by the slab allocation and the number of objs it can store:

# mem_cgroup,2368 size

| ORDER | SIZE | NUM_OBJS | ORIGIN |
| ----------- | ------- | ---------------- | ---------- |
| 3            | 32KB | 13                | 8           |
| 2            | 16KB | 6                  | 4           |
| 1            | 8KB   | 3                  | 2           |
| 0            | 4KB   | 1                  | 1           |

# mem_cgroup_per_node,2944 size

| ORDER | SIZE | NUM_OBJS | ORIGIN |
| ----------- | ------- | ---------------- | ----------  |
| 3            | 32KB | 11                | 8            |
| 2            | 16KB | 5                  | 4            |
| 1            | 8KB   | 2                  | 2            |
| 0            | 4KB   | 1                  | 1            |

So, for mem_cgroup, if page order > 1, then have optimize; while mem_cgroup_per_node needs order 2. :)

>
> So there is a packing advantage to turning off HWCACHE_ALIGN (for the
> first slab; no difference for the second).  BUT!  Now you have cacheline
> aliasing between two objects, and that's probably bad.  It's the kind
> of performance problem that's really hard to see.

Yes, And I would like to learn, in what situations do you think HWCACHE UNALIGN might cause issues?

Could it be direct memory reclaim by multiple processes? Or multiple processes charging memory simultaneously?

>
> Anyway, you've gone from allocating 8 objects per 32KiB to allocating
> 13 objects per 32KiB, a 62% improvement in memory consumption.

Thanks, that's more clearer.

Huan