Re: [PATCH bpf-next] bpf/test_run: increase Page Pool's ptr_ring size in live frames mode

[Alexander Lobakin]

From: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
Date: Thu, 15 Feb 2024 00:02:27 +0100

> Toke Høiland-Jørgensen <toke@xxxxxxxxxx> writes:
> 
>> Alexander Lobakin <aleksander.lobakin@xxxxxxxxx> writes:
>>
>>> Currently, when running xdp-trafficgen, test_run creates page_pools with
>>> the ptr_ring size of %NAPI_POLL_WEIGHT (64).
>>> This might work fine if XDP Tx queues are polled with the budget
>>> limitation. However, we often clear them with no limitation to ensure
>>> maximum free space when sending.
>>> For example, in ice and idpf (upcoming), we use "lazy" cleaning, i.e. we
>>> clean XDP Tx queue only when the free space there is less than 1/4 of
>>> the queue size. Let's take the ring size of 512 just as an example. 3/4
>>> of the ring is 384 and often times, when we're entering the cleaning
>>> function, we have this whole amount ready (or 256 or 192, doesn't
>>> matter).
>>> Then we're calling xdp_return_frame_bulk() and after 64th frame,
>>> page_pool_put_page_bulk() starts returning pages to the page allocator
>>> due to that the ptr_ring is already full. put_page(), alloc_page() et at
>>> starts consuming a ton of CPU time and leading the board of the perf top
>>> output.
>>>
>>> Let's not limit ptr_ring to 64 for no real reason and allow more pages
>>> to be recycled. Just don't put anything to page_pool_params::size and
>>> let the Page Pool core pick the default of 1024 entries (I don't believe
>>> there are real use cases to clean more than that amount of descriptors).
>>> After the change, the MM layer disappears from the perf top output and
>>> all pages get recycled to the PP. On my test setup on idpf with the
>>> default ring size (512), this gives +80% of Tx performance with no
>>> visible memory consumption increase.
>>>
>>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
>>
>> Hmm, so my original idea with keeping this low was to avoid having a lot
>> of large rings lying around if it is used by multiple processes at once.
>> But we need to move away from the per-syscall allocation anyway, and
>> with Lorenzo's patches introducing a global system page pool we have an
>> avenue for that. So in the meantime, I have no objection to this...
>>
>> Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
> 
> Actually, since Lorenzo's patches already landed in net-next, let's just
> move to using those straight away. I'll send a patch for this tomorrow :)

Keep in mind that system page_pools do direct recycling based on cpuid
and for now, memory leaks are possible. Pls see my patch[0] for the
details :D

> 
> -Toke
> 

[0]
https://lore.kernel.org/netdev/20240215113905.96817-1-aleksander.lobakin@xxxxxxxxx

Olek