Date: Tue, 30 Apr 2024 12:55:05 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Mina Almasry <almasrymina@...gle.com>
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Subject: Re: [RFC PATCH net-next v8 07/14] page_pool: devmem support

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? 

> 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.

-- 
Jens Axboe

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