From: Andrew Lunn <andrew@lunn.ch>
To: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Cc: David Miller <davem@davemloft.net>,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
kernel@savoirfairelinux.com, f.fainelli@gmail.com
Subject: Re: [PATCH net-next 1/7] net: dsa: hide dsa_uses_tagged_protocol code
Date: Tue, 30 May 2017 18:12:49 +0200 [thread overview]
Message-ID: <20170530161249.GK22758@lunn.ch> (raw)
In-Reply-To: <87wp8y2v9t.fsf@weeman.i-did-not-set--mail-host-address--so-tickle-me>
On Tue, May 30, 2017 at 11:56:30AM -0400, Vivien Didelot wrote:
> Hi Andrew, David,
>
> David Miller <davem@davemloft.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?
The additional dereference could cause a cache miss when accessing
tag_ops, which is expensive. dst will be in cache, so dst->rcv should
always be cheap.
Andrew
next prev parent reply other threads:[~2017-05-30 16:12 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-30 14:21 [PATCH net-next 0/7] net: dsa: tagger simplification Vivien Didelot
2017-05-30 14:21 ` [PATCH net-next 1/7] net: dsa: hide dsa_uses_tagged_protocol code Vivien Didelot
2017-05-30 15:01 ` Andrew Lunn
2017-05-30 15:09 ` David Miller
2017-05-30 15:56 ` Vivien Didelot
2017-05-30 16:12 ` Andrew Lunn [this message]
2017-05-30 16:14 ` Vivien Didelot
2017-05-30 21:08 ` kbuild test robot
2017-05-30 14:21 ` [PATCH net-next 2/7] net: dsa: remove useless rcv copy in DSA tree Vivien Didelot
2017-05-30 15:05 ` Andrew Lunn
2017-05-30 15:09 ` David Miller
2017-05-30 14:21 ` [PATCH net-next 3/7] net: dsa: remove unused arguments of tagger rcv Vivien Didelot
2017-05-30 15:13 ` Andrew Lunn
2017-05-30 14:21 ` [PATCH net-next 4/7] net: dsa: free orig skb on rcv if reallocated Vivien Didelot
2017-05-30 15:25 ` Andrew Lunn
2017-05-30 15:41 ` Vivien Didelot
2017-05-30 16:26 ` Andrew Lunn
2017-05-30 14:21 ` [PATCH net-next 5/7] net: dsa: remove useless goto label in tagger rcv Vivien Didelot
2017-05-30 14:21 ` [PATCH net-next 6/7] net: dsa: remove useless copy of tagger xmit Vivien Didelot
2017-05-30 15:29 ` Andrew Lunn
2017-05-30 15:43 ` Vivien Didelot
2017-05-30 14:21 ` [PATCH net-next 7/7] net: dsa: factor skb freeing on xmit Vivien Didelot
2017-05-30 15:37 ` Andrew Lunn
2017-05-30 15:08 ` [PATCH net-next 0/7] net: dsa: tagger simplification Andrew Lunn
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