linux-kernel - Re: [PATCH v4 3/3] powerpc/powernv: Introduce sysfs control for fastsleep workaround behavior

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Message-ID: <552CACD2.5060600@linux.vnet.ibm.com>
Date:	Tue, 14 Apr 2015 11:29:46 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	"Shreyas B. Prabhu" <shreyas@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>
CC:	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 3/3] powerpc/powernv: Introduce sysfs control for fastsleep
 workaround behavior

Hi Shreyas,

On 04/14/2015 07:26 AM, Shreyas B. Prabhu wrote:
> Fastsleep is one of the idle state which cpuidle subsystem currently
> uses on power8 machines. In this state L2 cache is brought down to a
> threshold voltage. Therefore when the core is in fastsleep, the
> communication between L2 and L3 needs to be fenced. But there is a bug
> in the current power8 chips surrounding this fencing.
> 
> OPAL provides a workaround which precludes the possibility of hitting
> this bug. But running with this workaround applied causes checkstop
> if any correctable error in L2 cache directory is detected. Hence OPAL
> also provides a way to undo the workaround.
> 
> In the existing implementation, workaround is applied by the last thread
> of the core entering fastsleep and undone by the first thread waking up.
> But this has a performance cost. These OPAL calls account for roughly
> 4000 cycles everytime the core has to enter or wakeup from fastsleep.
> 
> This patch introduces a sysfs attribute (fastsleep_workaround_state)
> to choose the behavior of this workaround.
> 
> By default, fastsleep_workaround_state = dynamic. In this case, workaround
> is applied/undone everytime the core enters/exits fastsleep.
> 
> fastsleep_workaround_state = applyonce. In this case the workaround is
> applied once on all the cores and never undone. This can be triggered by
> echo applyonce > /sys/devices/system/cpu/fastsleep_workaround_state

I was wondering if we really need such an elaborate design for this
sysfs file. Why not a sysfs file called fastsleep_workaround_apply_once,
which is set to '0' by default and the only value that it can take is
'1' ? The name easily implies that the workaround is applied only once
if it is set. I can see that this can cut down a good chunk of code from
this patch. I just didn't find too much value in having so much code for
a simple 'on' knob.

Regards
Preeti U Murthy

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