netdev - Re: [PATCH v2] net: cache for same cpu skb_attempt_defer

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Message-ID: <d8948716-ed07-48ab-a933-671f1fc4ee58@gmail.com>
Date: Wed, 14 Feb 2024 16:37:41 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
 davem@...emloft.net, dsahern@...nel.org, pabeni@...hat.com
Subject: Re: [PATCH v2] net: cache for same cpu skb_attempt_defer_free

On 2/14/24 03:13, Jakub Kicinski wrote:
> On Tue, 13 Feb 2024 14:07:24 +0000 Pavel Begunkov wrote:
>>>> +       local_bh_disable();
>>>> +       skb_release_all(skb, SKB_DROP_REASON_NOT_SPECIFIED, false);
>>>
>>> I am trying to understand why we use false instead of true here ?
>>> Or if you prefer:
>>> local_bh_disable();
>>> __napi_kfree_skb(skb, SKB_DROP_REASON_NOT_SPECIFIED);
>>> local_bh_enable();
> 
> FWIW I had the same reaction. napi_safe = false followed by
> napi_skb_cache_put() looks sus. No argument that naming is bad,
> not the first time it comes up :(
> 
>> Maybe it's my misunderstanding but disabled bh != "napi safe",
>> e.g. the napi_struct we're interested in might be scheduled for
>> another CPU. Which is also why "napi" prefix in percpu
>> napi_alloc_cache sounds a bit misleading to me.
> 
> FWIW the skb recycling is called napi_* to hint to driver authors that
> if they are in NAPI context this is a better function to call.

Which is absolutely reasonable, napi_skb_cache_put() on the
other hand is rather internal and wouldn't be used by drivers
directly.
I guess I'll just do a little bit of renaming later hopefully after
this patch is taken in, unless there are other comments / objections.

> The connection to a particular NAPI instance matters only for the page
> pool recycling, but that's handled. The conditions you actually
> need to look out for are hardware IRQs and whatever async paths which
> can trigger trigger while NAPI is half way thru touching the cache of
> the local CPU.

Yeah the usual percpu (bh protected) caching stuff and likes, and
the question is rather about the semantics.

To draw some conclusion, do you suggest to change anything
in the patch?

>> The second reason is that it shouldn't change anything
>> performance wise
>>
>> napi_pp_put_page(napi_safe) {
>>       ...
>>       if (napi_safe || in_softirq()) { ... }
>> }

-- 
Pavel Begunkov