linux-kernel - Re: [PATCH bisected regression] sched: rebuild sched domains at suspend/resume

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Date:	Wed, 7 Mar 2012 15:02:46 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	prashanth@...ux.vnet.ibm.com, "Rafael J. Wysocki" <rjw@...k.pl>,
	Linux PM mailing list <linux-pm@...r.kernel.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	"paulmck@...ux.vnet.ibm.com >> \"Paul E. McKenney\"" 
	<paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH bisected regression] sched: rebuild sched domains at
 suspend/resume


* Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com> wrote:

> On 03/07/2012 02:15 AM, Konstantin Khlebnikov wrote:
> 
> > This is fix for suspend/resume regression introduced in 
> > commit 8f2f748b0656 ("CPU hotplug, cpusets, suspend: Don't 
> > touch cpusets during suspend/resume") Without this patch 
> > suspend always hangs on my thinkpad x220 (2 x CPU * HT).
> 
> 
> Hey, with commit 8f2f748b0656, suspend/resume works perfectly 
> for me! I ran it multiple times just to make sure, and 
> everything worked just great.
>
> Apart from that, I even tried suspend/resume after building 
> the kernel with and without CONFIG_CPUSETS. Both cases worked 
> perfectly.
> 
> So, I am really surprised at what you stated above. Are you 
> *really* sure you are facing suspend hangs *because* of the 
> above commit?

That's not the only information Konstantin gave: he also 
provided a patch, which activates a side-effect that got removed 
by your patch - and voila, s2ram was working for him again.

At this point I don't think we should doubt the messenger, we 
should doubt our assumptions and we should go for a clean revert 
ASAP.

I don't see the underlying bug straight away, but clearly my 
assumptions of this patch not causing problems, when I applied 
the patch, are not valid anymore.

So if Linus wants to revert the commit straight away:

Acked-by: Ingo Molnar <mingo@...e.hu>

> And AFAICS hardware doesn't matter for the code in question, but in any case,
> the laptop on which I tested it is:
> Thinkpad T420 (Intel core i5-2540M), 2 cores * HT (total 4 logical cpus).

Well, any environmental detail might matter as long as we don't 
understand the bug.

> Also, the patch you posted here doesn't make much sense.. nor 
> does it give a clue as to what might be wrong at your end (if 
> anything is really wrong, that is). Do you have CONFIG_CPUSETS 
> set or unset? Could you share your .config?

His patch is simply the re-creation of a side-effect that the 
kernel had before - and it made suspend work for him. While it 
is not a patch we want to apply, it sure was a valid and 
valuable experiment to perform.

Thanks,

	Ingo
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