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Message-ID: <ZagQGZ5CM3vEH2RP@lore-desk>
Date: Wed, 17 Jan 2024 18:36:25 +0100
From: Lorenzo Bianconi <lorenzo@...nel.org>
To: Jesper Dangaard Brouer <hawk@...nel.org>, kuba@...nel.org
Cc: Paolo Abeni <pabeni@...hat.com>,
Ilias Apalodimas <ilias.apalodimas@...aro.org>,
netdev@...r.kernel.org,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
willemdebruijn.kernel@...il.com, toke@...hat.com,
davem@...emloft.net, edumazet@...gle.com, bpf@...r.kernel.org,
lorenzo.bianconi@...hat.com, sdf@...gle.com, jasowang@...hat.com
Subject: Re: [PATCH v5 net-next 1/3] net: introduce page_pool pointer in
softnet_data percpu struct
>
>
> On 19/12/2023 16.23, Paolo Abeni wrote:
> > On Thu, 2023-12-14 at 15:29 +0100, Lorenzo Bianconi wrote:
> > > Allocate percpu page_pools in softnet_data.
> > > Moreover add cpuid filed in page_pool struct in order to recycle the
> > > page in the page_pool "hot" cache if napi_pp_put_page() is running on
> > > the same cpu.
> > > This is a preliminary patch to add xdp multi-buff support for xdp running
> > > in generic mode.
> > >
> > > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> > > ---
> > > include/linux/netdevice.h | 1 +
> > > include/net/page_pool/helpers.h | 5 +++++
> > > include/net/page_pool/types.h | 1 +
> > > net/core/dev.c | 39 ++++++++++++++++++++++++++++++++-
> > > net/core/page_pool.c | 5 +++++
> > > net/core/skbuff.c | 5 +++--
> > > 6 files changed, 53 insertions(+), 3 deletions(-)
> >
> > @Jesper, @Ilias: could you please have a look at the pp bits?
> >
>
> I have some concerns... I'm still entertaining the idea, but we need to
> be aware of the tradeoffs we are making.
>
> (1)
> Adding PP to softnet_data means per CPU caching 256 pages in the
> ptr_ring (plus likely 64 in the alloc-cache). Fortunately, PP starts
> out empty, so as long as this PP isn't used they don't get cached. But
> if used, then PP don't have a MM shrinker that removes these cached
> pages, in case system is under MM pressure. I guess, you can argue that
> keeping this per netdev rx-queue would make memory usage even higher.
> This is a tradeoff, we are trading memory (waste) for speed.
>
>
> (2) (Question to Jakub I guess)
> How does this connect with Jakub's PP netlink stats interface?
> E.g. I find it very practical that this allow us get PP stats per
> netdev, but in this case there isn't a netdev.
>
>
> (3) (Implicit locking)
> PP have lockless "alloc" because it it relies on drivers NAPI context.
> The places where netstack access softnet_data provide similar protection
> that we can rely on for PP, so this is likely correct implementation
> wise. But it will give people like Sebastian (Cc) more gray hair when
> figuring out how PREEMPT_RT handle these cases.
>
> (4)
> The optimization is needed for the case where we need to re-allocate and
> copy SKB fragments. I think we should focus on avoiding this code path,
> instead of optimizing it. For UDP it should be fairly easy, but for TCP
> this is harder.
Hi all,
I would resume this activity and it seems to me there is no a clear direction
about where we should add the page_pool (in a per_cpu pointer or in
netdev_rx_queue struct) or if we can rely on page_frag_cache instead.
@Jakub: what do you think? Should we add a page_pool in a per_cpu pointer?
Regards,
Lorenzo
>
> --Jesper
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