linux-kernel - Re: [RFC PATCH net-next v8 07/14] page

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Date: Tue, 30 Apr 2024 12:19:17 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Jens Axboe <axboe@...nel.dk>
Cc: David Wei <dw@...idwei.uk>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
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	Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC PATCH net-next v8 07/14] page_pool: devmem support

On Tue, Apr 30, 2024 at 11:55 AM Jens Axboe <axboe@...nel.dk> wrote:
>
> On 4/30/24 12:29 PM, Mina Almasry wrote:
> > On Tue, Apr 30, 2024 at 6:46?AM Jens Axboe <axboe@...nel.dk> wrote:
> >>
> >> On 4/26/24 8:11 PM, Mina Almasry wrote:
> >>> On Fri, Apr 26, 2024 at 5:18?PM David Wei <dw@...idwei.uk> wrote:
> >>>>
> >>>> On 2024-04-02 5:20 pm, Mina Almasry wrote:
> >>>>> @@ -69,20 +106,26 @@ net_iov_binding(const struct net_iov *niov)
> >>>>>   */
> >>>>>  typedef unsigned long __bitwise netmem_ref;
> >>>>>
> >>>>> +static inline bool netmem_is_net_iov(const netmem_ref netmem)
> >>>>> +{
> >>>>> +#if defined(CONFIG_PAGE_POOL) && defined(CONFIG_DMA_SHARED_BUFFER)
> >>>>
> >>>> I am guessing you added this to try and speed up the fast path? It's
> >>>> overly restrictive for us since we do not need dmabuf necessarily. I
> >>>> spent a bit too much time wondering why things aren't working only to
> >>>> find this :(
> >>>
> >>> My apologies, I'll try to put the changelog somewhere prominent, or
> >>> notify you when I do something that I think breaks you.
> >>>
> >>> Yes, this is a by-product of a discussion with regards to the
> >>> page_pool benchmark regressions due to adding devmem. There is some
> >>> background on why this was added and the impact on the
> >>> bench_page_pool_simple tests in the cover letter.
> >>>
> >>> For you, I imagine you want to change this to something like:
> >>>
> >>> #if defined(CONFIG_PAGE_POOL)
> >>> #if defined(CONFIG_DMA_SHARED_BUFFER) || defined(CONFIG_IOURING)
> >>>
> >>> or something like that, right? Not sure if this is something I should
> >>> do here or if something more appropriate to be in the patches you
> >>> apply on top.
> >>
> >> In general, attempting to hide overhead behind config options is always
> >> a losing proposition. It merely serves to say "look, if these things
> >> aren't enabled, the overhead isn't there", while distros blindly enable
> >> pretty much everything and then you're back where you started.
> >>
> >
> > The history there is that this check adds 1 cycle regression to the
> > page_pool fast path benchmark. The regression last I measured is 8->9
> > cycles, so in % wise it's a quite significant 12.5% (more details in
> > the cover letter[1]). I doubt I can do much better than that to be
> > honest.
>
> I'm all for cycle counting, and do it myself too, but is that even
> measurable in anything that isn't a super targeted microbenchmark? Or
> even in that?
>

Not as far as I can tell, no. This was purely to improve the page_pool
benchmark.

> > There was a desire not to pay this overhead in setups that will likely
> > not care about devmem, like embedded devices maybe, or setups without
> > GPUs. Adding a CONFIG check here seemed like very low hanging fruit,
> > but yes it just hides the overhead in some configs, not really removes
> > it.
> >
> > There was a discussion about adding this entire netmem/devmem work
> > under a new CONFIG. There was pushback particularly from Willem that
> > at the end of the day what is enabled on most distros is what matters
> > and we added code churn and CONFIG churn for little value.
> >
> > If there is significant pushback to the CONFIG check I can remove it.
> > I don't feel like it's critical, it just mirco-optimizes some setups
> > that doesn't really care about this work area.
>
> That is true, but in practice it'll be enabled anyway. Seems like it's
> not really worth it in this scenario.
>

OK, no pushback from me. I'll remove the CONFIG check in the next iteration.

-- 
Thanks,
Mina