linux-kernel - Re: [PATCH] Revert 9fc2105aeaaf56b0cf75296a84702d0f9e64437b to fix pyaudio (and probably more)

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Message-ID: <20150105123252.GB10116@arm.com>
Date:	Mon, 5 Jan 2015 12:32:52 +0000
From:	Will Deacon <will.deacon@....com>
To:	Theodore Ts'o <tytso@....edu>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pavel Machek <pavel@....cz>,
	Marc Zyngier <marc.zyngier@....com>,
	kernel list <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] Revert 9fc2105aeaaf56b0cf75296a84702d0f9e64437b to fix
 pyaudio (and probably more)

Hi Ted,

On Mon, Jan 05, 2015 at 01:34:36AM +0000, Theodore Ts'o wrote:
> On Sun, Jan 04, 2015 at 09:26:59PM +0000, Russell King - ARM Linux wrote:
> > 
> > With the revert in place, we now have insanely small bogomips values
> > reported via /proc/cpuinfo when hardware timers are used.  That needs
> > fixing.
> 
> Why does it need to be fixed?
> 
> It's clear that there are applications that are working OK with the
> existing value,

I'm not sure it is that clear -- the reported regression was on a processor
that doesn't use the timer-backed delay loop, so the bogomips value will
essentially be restored by reverting the patch.

The issue comes on newer CPUs, where there will now be a very small bogomips
value reported and (to my knowledge) nobody has yet tried running some
affected applications there to see if they can cope.

> and if you change it to fix it for some new applications, but it breaks
> for others, then have you considered defining a new interface (perhaps
> exported via sysfs) that exports a "sane" value and document that new
> applications shoud use the new interface.
> 
> Or if the answer is that no one should be using the bogomips field at
> all, then just document *that*, and then leave it be, so that existing
> applications don't break.

It never hurts to document our assumptions or anticipated/preferred use-cases
but in this case I think bogomips is difficult enough to use on any
half-recent SoCs that most developers have either (a) found another way to
do what they want (perf counters, clock_gettime) or (b) stopped bothering to
guess the CPU frequency when it's not actually needed, so I don't *think*
that new applications are such an issue.

Cheers,

Will
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