netdev - Re: [net-next PATCH V1 3/3] net: warn on napi_alloc

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Message-ID: <50c0ae0c-2382-42ea-998b-f5c641e5db4a@cogentembedded.com>
Date:	Mon, 9 May 2016 16:53:50 +0300
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>
Cc:	saeedm@...lanox.com, gerlitz.or@...il.com, eugenia@...lanox.com,
	Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [net-next PATCH V1 3/3] net: warn on napi_alloc_skb being called
 in wrong context

Hello.

On 5/9/2016 4:44 PM, Jesper Dangaard Brouer wrote:

> It have always been required to call napi_alloc_skb from NAPI/softirq

    It has.

> context, which implies running with local_bh_disable'ed.  Thus, this

    Not local_bh_disable'd?

> code path should already be well tested. But recent SKB bulk changes
> introduced will make this more volatile and bugs more subtle, if this
> is violated.
>
> To catch any driver violating this add a loud WARN_ON.
>
> Performance wise, I do worry about adding this runtime check code into
> the hotpath, of this highly optimized function call.  I've

    Hot path? My spellchecker trips here.

> micro-benchmarked it with both IP-forwarding and local UDP delivery,
> and didn't see any regressions.  It does adds extra code size (icache).
>
> add/remove: 0/0 grow/shrink: 1/0 up/down: 43/0 (43)
> function                                     old     new   delta
> __napi_alloc_skb                             461     504     +43
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
[...]

MBR, Sergei