linux-kernel - Re: [RFC][PATCH] ring-buffer: Replace this_cpu_{read,write} with this_cpu

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Message-ID: <alpine.DEB.2.11.1503170049300.9309@gentwo.org>
Date:	Tue, 17 Mar 2015 00:56:51 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	linux-arm-kernel@...ts.infradead.org,
	Russell King <linux@....linux.org.uk>
Subject: Re: [RFC][PATCH] ring-buffer: Replace this_cpu_{read,write} with
 this_cpu_ptr()

On Mon, 16 Mar 2015, Steven Rostedt wrote:

> It has come to my attention that this_cpu_read/write are horrible on
> architectures other than x86. Worse yet, they actually disable
> preemption or interrupts! This caused some unexpected tracing results
> on ARM.

Well its just been 7 years or so. Took a long time it seems.

These would need to be implemented on the architectures to
have comparable performance.

> I may go and remove all this_cpu_read,write() calls from my code
> because of this.

You could do that with __this_cpo_* but not this_cpu_*(). Doing
it to this_cpu_* would make the operations no longer per cpu atomic. If
they do not need per cpu atomicity then you could have used __this_cpu_*
instead. And  __this_cpu_* do not disable preemption or interrupts.

So please do not send patches based on gut reactions.

NAK

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