Re: [PATCH net-next] sock: don't enable netstamp for af_unix sockets

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

On Tue, Oct 27, 2015, at 12:09, Hannes Frederic Sowa wrote:
> Hi Richard,
> 
> On Tue, Oct 27, 2015, at 11:11, Richard Cochran wrote:
> > On Mon, Oct 26, 2015 at 02:32:59PM +0100, Hannes Frederic Sowa wrote:
> > > On Mon, Oct 26, 2015, at 14:19, Richard Cochran wrote:
> > > > On Mon, Oct 26, 2015 at 01:51:37PM +0100, Hannes Frederic Sowa wrote:
> > > > > netstamp_needed is toggled for all socket families if they request
> > > > > timestamping. But some protocols don't need the lower-layer timestamping
> > > > > code at all. This patch starts disabling it for af-unix.
> > > > 
> > > > What problem is this patch trying to solve?
> > > 
> > > netstamp_needed is a static-key which enables timestamping code in the
> > > networking stack receive functions for every packet, while it is not
> > > needed for AF_UNIX/LOCAL. So it is merely a small performance
> > > enhancement.
> > 
> > Are there any numbers that show the effect of this enhancement?
> 
> I haven't personally done any performance numbers.
> 
> Jesper (in Cc) noticed that it showed up in perf performance reports
> even though he used a very minimal setup. Turned out that
> systemd-journald enables timestamping on AF_UNIX sockets which thus
> enabled netstamps globally. I think Jesper can chime in here.

Also counter question: why is the netstamp code protected by a
static_key otherwise if not for trying to suppress the code path as
often as possible if not used? ;)

Bye,
Hannes