netdev - Re: [PATCH net] net: avoid 32 x truesize under-estimation for tiny skbs

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Message-ID: <66c8b7c2-25a6-2834-b341-22b6498e3f7e@gmail.com>
Date:   Wed, 7 Sep 2022 13:40:40 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Alexander Duyck <alexanderduyck@...com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH net] net: avoid 32 x truesize under-estimation for tiny
 skbs


On 9/7/22 13:19, Paolo Abeni wrote:
> Hello,
>
> reviving an old thread...
> On Wed, 2021-01-13 at 08:18 -0800, Eric Dumazet wrote:
>> While using page fragments instead of a kmalloc backed skb->head might give
>> a small performance improvement in some cases, there is a huge risk of
>> under estimating memory usage.
> [...]
>
>> Note that we might in the future use the sk_buff napi cache,
>> instead of going through a more expensive __alloc_skb()
>>
>> Another idea would be to use separate page sizes depending
>> on the allocated length (to never have more than 4 frags per page)
> I'm investigating a couple of performance regressions pointing to this
> change and I'd like to have a try to the 2nd suggestion above.
>
> If I read correctly, it means:
> - extend the page_frag_cache alloc API to allow forcing max order==0
> - add a 2nd page_frag_cache into napi_alloc_cache (say page_order0 or
> page_small)
> - in __napi_alloc_skb(), when len <= SKB_WITH_OVERHEAD(1024), use the
> page_small cache with order 0 allocation.
> (all the above constrained to host with 4K pages)
>
> I'm not quite sure about the "never have more than 4 frags per page"
> part.
>
> What outlined above will allow for 10 min size frags in page_order0, as
> (SKB_DATA_ALIGN(0) + SKB_DATA_ALIGN(struct skb_shared_info) == 384. I'm
> not sure that anything will allocate such small frags.
> With a more reasonable GRO_MAX_HEAD, there will be 6 frags per page.


Well, some arches have PAGE_SIZE=65536 :/


>
> The maximum truesize underestimation in both cases will be lower than
> what we can get with the current code in the worst case (almost 32x
> AFAICS).
>
> Is the above schema safe enough or should the requested size
> artificially inflatted to fit at most 4 allocations per page_order0?
> Am I miss something else? Apart from omitting a good deal of testing in
> the above list ;)
>
> Thanks!
>
> Paolo
>