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

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Date: Wed, 3 May 2023 17:49:34 +0200
From: Jesper Dangaard Brouer <jbrouer@...hat.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>,
 Jakub Kicinski <kuba@...nel.org>
Cc: brouer@...hat.com, Ilias Apalodimas <ilias.apalodimas@...aro.org>,
 netdev@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>,
 linux-mm@...ck.org, Mel Gorman <mgorman@...hsingularity.net>,
 lorenzo@...nel.org, linyunsheng@...wei.com, bpf@...r.kernel.org,
 "David S. Miller" <davem@...emloft.net>, Paolo Abeni <pabeni@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>, willy@...radead.org
Subject: Re: [PATCH RFC net-next/mm V3 1/2] page_pool: Remove workqueue in new
 shutdown scheme

On 03/05/2023 13.18, Toke Høiland-Jørgensen wrote:
> Jakub Kicinski <kuba@...nel.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? :)

I agree that this workqueue that keeps rescheduling is wasteful.
It actually reschedules every second, even more wasteful.
NIC drivers will have many HW RX-queues, with separate PP instances, 
that each can start a workqueue that resched every sec.

Eric have convinced me that SKBs can "stick around" for longer than the
assumptions in PP.  The old PP assumptions came from XDP-return path.
It is time to cleanup.

> 
> 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...

Yes, the XDP live packet mode of bpf_prog_run is IMHO abusing the
page_pool API.  We should fix that somehow, at least the case where live
packet mode is only injecting a single packet, but still creates a PP
instance. The PP in live packet mode IMHO only makes sense when
repeatedly sending packets that gets recycles and are pre-inited by PP.

This use of PP does exemplify why is it problematic to keep the workqueue.

I have considered (and could be convinced) delaying the free via
call_rcu, but it also create an unfortunate backlog of work in the case
of live packet mode of bpf_prog_run.

--Jesper