Re: [PATCH v2] cfg80211: Don't re-use the skb for larger NL messages.

[Johannes Berg <johannes@sipsolutions.net>]

On Sun, 2015-05-31 at 16:47 +0530, Krishna Chaitanya wrote:

> > Note # of "messages" as you say is actually irrelevant - you should look
> 
> Well with 138 messages the function and each message 3072 bytes
> calls b/w cfg80211 and driver
>        without the patch: would be 276 calls
>        with this patch: would be 138 calls
> Thats a lot of function calls, don't you think?

No, I don't think so. That really should be within the noise, it's all
in the icache already after the first round.

> > at how often the kernel/user boundary is crossed, that's really far more
> > interesting, and your patch makes that MUCH worse when the put size is
> > small (say 100 bytes) because then you're practically doing that twice
> > as often.
> 
> My patch doesn't deteriorate the situation, and not change the kernel to
> user boundary. With/Without the patch 3072 bytes are transported in a
> single message from kernel to user.

*in your case*

In the case that somebody is creating smaller messages it makes things
MUCH worse by allowing only half the data to be carried across the
kernel/userspace boundary each time any data crosses it, so it will
result in many more syscalls in that case. If you're worried about the
overhead of a simple function (pointer) call in the kernel, then surely
you should be far more worried about this.

johannes