Re: [PATCH RFC net-next/mm V3 1/2] page_pool: Remove workqueue in new shutdown scheme

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Jakub Kicinski <kuba@kernel.org> writes:

> On Fri, 28 Apr 2023 18:16:19 +0200 Jesper Dangaard Brouer wrote:
>> This removes the workqueue scheme that periodically tests when
>> inflight reach zero such that page_pool memory can be freed.
>> 
>> This change adds code to fast-path free checking for a shutdown flags
>> bit after returning PP pages.
>
> We can remove the warning without removing the entire delayed freeing
> scheme. I definitely like the SHUTDOWN flag and patch 2 but I'm a bit
> less clear on why the complexity of datapath freeing is justified.
> Can you explain?

You mean just let the workqueue keep rescheduling itself every minute
for the (potentially) hours that skbs will stick around? Seems a bit
wasteful, doesn't it? :)

We did see an issue where creating and tearing down lots of page pools
in a short period of time caused significant slowdowns due to the
workqueue mechanism. Lots being "thousands per second". This is possible
using the live packet mode of bpf_prog_run() for XDP, which will setup
and destroy a page pool for each syscall...

-Toke