Re: [PATCH net] net: page_pool: use in_softirq() instead

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 03/02/2023 14.05, DENG Qingfang wrote:
> Hi Jesper,
> 
> On Fri, Feb 3, 2023 at 7:15 PM Jesper Dangaard Brouer
> <jbrouer@redhat.com> wrote:
>> How can I enable threaded NAPI on my system?
> 
> dev_set_threaded(napi_dev, true);
> 
> You can also enable it at runtime by writing 1 to
> /sys/class/net/<devname>/threaded, but it only works if the driver
> does _not_ use a dummy netdev for NAPI poll.
> 

Thanks for providing this setup info.

I quickly tested driver i40e with a XDP_DROP workload, and switch 
between the threaded and normal NAPI, and no performance difference.
(p.s. driver i40e doesn't use page_pool)

>> I think other cases (above) are likely safe, but I worry a little about
>> this case, as the page_pool_recycle_in_cache() rely on RX-NAPI protection.
>> Meaning it is only the CPU that handles RX-NAPI for this RX-queue that
>> is allowed to access this lockless array.
> 
> The major changes to the threaded NAPI is that instead of scheduling a
> NET_RX softirq, it wakes up a kthread which then does the polling,
> allowing it to be scheduled to another CPU. The driver's poll function
> is called with BH disabled so it's still considered BH context.
> 

As long as drivers NAPI poll function doesn't migrate between CPUs while
it is running this should be fine. (This guarantee is needed as XDP + TC
have per_cpu bpf_redirect_info).

Looking at the code napi_threaded_poll() in net/core/dev.c I can see
this is guarantee is provided by the local_bh_disable() +
local_bh_enable around the call to __napi_poll().

>> We do have the 'allow_direct' boolean, and if every driver/user uses
>> this correctly, then this should be safe.  Changing this makes it
>> possible for drivers to use page_pool API incorrectly and this leads to
>> hard-to-debug errors.
> 
> "incorrectly", like, calling it outside RX-NAPI?

Yes.

--Jesper