linux-kernel - Re: [PATCH net-next 1/7] net: dsa: hide dsa_uses_tagged

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Date:   Tue, 30 May 2017 11:56:30 -0400
From:   Vivien Didelot <vivien.didelot@...oirfairelinux.com>
To:     David Miller <davem@...emloft.net>, andrew@...n.ch
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...oirfairelinux.com, f.fainelli@...il.com
Subject: Re: [PATCH net-next 1/7] net: dsa: hide dsa_uses_tagged_protocol code

Hi Andrew, David,

David Miller <davem@...emloft.net> writes:

>>> +bool dsa_uses_tagged_protocol(struct dsa_switch_tree *dst)
>>> +{
>>> +	return !!dst->rcv;
>>> +}
>>> +
>> 
>> You need to be careful here. This is in the hot path. Every frame
>> received uses this code. And think about a distro kernel, which might
>> have DSA enabled by default, yet is unlikely to have any switches. You
>> are adding a function call which can be called millions of times per
>> second....
>
> Yeah, we really can't make this change.
>
> This isn't glibc where we're trying to hide the implementation of "FILE *"
> behind accessor functions that caller can't see.  We inline things when
> performance dictates, and it does here.

Thanks for the explanation, this wasn't obvious to me at all. So inline
is mandatory here. Would a dereference like "!!dst->tag_ops->rcv" have
an significant impact on performance?

Thanks,

        Vivien