netdev - Re: [PATCH net-next 2/2] udp: implement and use per cpu rx skbs cache

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Date:   Mon, 23 Apr 2018 11:13:16 +0300
From:   Tariq Toukan <tariqt@...lanox.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Tariq Toukan <tariqt@...lanox.com>
Subject: Re: [PATCH net-next 2/2] udp: implement and use per cpu rx skbs cache



On 20/04/2018 4:48 PM, Jesper Dangaard Brouer wrote:
> 
> On Thu, 19 Apr 2018 06:47:10 -0700 Eric Dumazet <eric.dumazet@...il.com> wrote:
>> On 04/19/2018 12:40 AM, Paolo Abeni wrote:
>>> On Wed, 2018-04-18 at 12:21 -0700, Eric Dumazet wrote:
>>>> On 04/18/2018 10:15 AM, Paolo Abeni wrote:
> [...]
>>>
>>> Any suggestions for better results are more than welcome!
>>
>> Yes, remote skb freeing. I mentioned this idea to Jesper and Tariq in
>> Seoul (netdev conference). Not tied to UDP, but a generic solution.
> 
> Yes, I remember.  I think... was it the idea, where you basically
> wanted to queue back SKBs to the CPU that allocated them, right?
> 
> Freeing an SKB on the same CPU that allocated it, have multiple
> advantages. (1) the SLUB allocator can use a non-atomic
> "cpu-local" (double)cmpxchg. (2) the 4 cache-lines memset cleared of
> the SKB stay local.  (3) the atomic SKB refcnt/users stay local.
> 
> We just have to avoid that queue back SKB's mechanism, doesn't cost
> more than the operations we expect to save.  Bulk transfer is an
> obvious approach.  For storing SKBs until they are returned, we already
> have a fast mechanism see napi_consume_skb calling _kfree_skb_defer,
> which SLUB/SLAB-bulk free to amortize cost (1).
> 
> I guess, the missing information is that we don't know what CPU the SKB
> were created on...
> 
> Where to store this CPU info?
> 
> (a) In struct sk_buff, in a cache-line that is already read on remote
> CPU in UDP code?
> 
> (b) In struct page, as SLUB alloc hand-out objects/SKBs on a per page
> basis, we could have SLUB store a hint about the CPU it was allocated
> on, and bet on returning to that CPU ? (might be bad to read the
> struct-page cache-line)
> 

I'm in favor of (a).

Pages of an SKB originates on the same cpu (guaranteed by NAPI).
So a single field in SKB is good for all of its fragments, no need to 
read this info from every single page. This also keeps the change local 
to the networking subsystem.

Best if we find a hole in struct SKB (for u16?), or union it with a 
mutually exclusive field.

Regards,
Tariq