linux-kernel - Re: [GIT PULL] ring-buffer: Replace this_cpu_*() with __this_cpu

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Date:	Tue, 19 May 2015 11:20:23 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	stable <stable@...r.kernel.org>,
	Uwe Kleine-Koenig <u.kleine-koenig@...gutronix.de>
Subject: Re: [GIT PULL] ring-buffer: Replace this_cpu_*() with
 __this_cpu_*()

On Tue, 19 May 2015, Steven Rostedt wrote:

> On Tue, 19 May 2015 10:35:32 -0500 (CDT)
> Christoph Lameter <cl@...ux.com> wrote:
>
> > On Thu, 19 Mar 2015, Steven Rostedt wrote:
> >
> > > On Thu, 19 Mar 2015 15:16:25 -0700
> > > Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > >
> > > > So I don't think the ring-buffer change is necessarily _wrong_, but if
> > > > this is a performance issue, why don't we just fix it up for the
> > > > generic case rather than for just one user?
> > >
> > > I totally agree with your analysis, but it's up to Christoph to come up
> > > with an answer to your questions.
> >
> > Something beyond: Do not use this_cpu_* when preemption is already
> > off but use __this_cpu_*?
>
> I think the question was, why exactly does the generic this_cpu_read()
> require disabling preemption? What breaks if it is not disabled?

Ok I answered that before. There is a cache line population/eviction
issue (because it may happen on the wrong cache) but basically I think
its mostly there for symmetries sake. We could remove
this_cpu_read/write and let __this_cpu_read()/__this_cpu_write be used in non
preemptible contexts.

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