linux-kernel - Re: [PATCH 09/11] pps: low level IRQ timestamps recording.

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Message-ID: <4B170204.4080604@zytor.com>
Date:	Wed, 02 Dec 2009 16:10:44 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Rodolfo Giometti <giometti@...ux.it>
CC:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Dave Jones <davej@...hat.com>, Sam Ravnborg <sam@...nborg.org>,
	Greg KH <greg@...ah.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Kay Sievers <kay.sievers@...y.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ingo Molnar <mingo@...e.hu>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 09/11] pps: low level IRQ timestamps recording.

On 12/02/2009 09:18 AM, Rodolfo Giometti wrote:
> Add low level IRQ timestamps recording for x86 (32 and 64 bits)
> platforms and enable UART clients in order to use it.
> 
> This improves PPS precision. :)

It also invokes getnstimeofday on every single interrupt, including ones
which have absolutely nothing to do with the PPS, and are potentially
high volume.  getnstimeofday can be a fairly expensive operation,
especially if the best available clock is away from the CPU, or it
requires extensive arithmetic in order to normalize the clock value...

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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