linux-kernel - Re: [PATCH v4 2/8] init.h: add __read_mostly to early_param_on

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Date:	Thu, 30 Apr 2015 13:46:22 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
cc:	Borislav Petkov <bp@...e.de>, Kyle McMartin <kyle@...nel.org>,
	Alok N Kataria <alokk@...softinc.com>,
	Shobhit Dayal <shobhit@...softinc.com>,
	Shai Fultheim <shai@...lex86.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Luis R. Rodriguez" <mcgrof@...not-panic.com>, mingo@...e.hu,
	tglx@...utronix.de, hpa@...or.com, plagnioj@...osoft.com,
	tomi.valkeinen@...com, daniel.vetter@...el.com, airlied@...ux.ie,
	dledford@...hat.com, awalls@...metrocast.net, syrjala@....fi,
	luto@...capital.net, mst@...hat.com, cocci@...teme.lip6.fr,
	linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Dave Airlie <airlied@...hat.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, x86@...nel.org
Subject: Re: [PATCH v4 2/8] init.h: add __read_mostly to
 early_param_on_off()

On Thu, 30 Apr 2015, Luis R. Rodriguez wrote:

> > But for every param declared this way? I'm not sure we want that...
>
> I'm glad you bring this up, Christoph can you or anyone else can you provide
> advise since you added __read_mostly ?

Well it should be reseved for data that is actually used frequently in hot
paths.  Initially we only moved variables into __read_mostly after we saw
in  performance traces that there was contention on an item because a
neighboring variable was frequently written to.

You want the __read_mostly data to be tighly packed. In the best case
multiple frequently read variables for a hot path will be next to each
other in order to reduce the number of cachelines needed to execute a
critical path. That means being selective and aware of which variables
actually benefit.
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