netdev - Re: [PATCH v3 bpf-next] net: veth: alloc skb in bulk for ndo_xdp

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Message-ID: <65a218bc-3ebf-001e-174d-b67817c83b45@iogearbox.net>
Date:   Thu, 4 Feb 2021 16:43:10 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, toshiaki.makita1@...il.com,
        lorenzo.bianconi@...hat.com, toke@...hat.com,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 bpf-next] net: veth: alloc skb in bulk for ndo_xdp_xmit

On 2/4/21 10:05 AM, Jesper Dangaard Brouer wrote:
[...]
> It was Andrew (AKPM) that wanted the API to either return the requested
> number of objects or fail. I respected the MM-maintainers request at
> that point, even-though I wanted the other API as there is a small
> performance advantage (not crossing page boundary in SLUB).
> 
> At that time we discussed it on MM-list, and I see his/the point:
> If API can allocate less objs than requested, then think about how this
> complicated the surrounding code. E.g. in this specific code we already
> have VETH_XDP_BATCH(16) xdp_frame objects, which we need to get 16 SKB
> objects for.  What should the code do if it cannot get 16 SKBs(?).

Right, I mentioned the error handling complications above wrt < n_skb case. I think iff this
ever gets implemented and there's a need, it would probably be best to add a new flag like
__GFP_BULK_BEST_EFFORT to indicate that it would be okay to return x elements with x being
in (0, size], so that only those callers need to deal with this, and all others can expect
[as today] that != 0 means all #size elements were bulk alloc'ed.

Thanks,
Daniel