linux-kernel - Re: Regression in 2.6.27-rc1 for set_cpus_allowed

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Date:	Mon, 11 Aug 2008 14:30:20 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Dmitry Adamushko <dmitry.adamushko@...il.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Langsdorf, Mark" <mark.langsdorf@....com>,
	lkml <linux-kernel@...r.kernel.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Regression in 2.6.27-rc1 for set_cpus_allowed_ptr


* Dmitry Adamushko <dmitry.adamushko@...il.com> wrote:

> 2008/8/8 Rafael J. Wysocki <rjw@...k.pl>:
> > [Adding CCs]
> >
> > On Friday, 8 of August 2008, Langsdorf, Mark wrote:
> >> One of my co-workers noticed that the powernow-k8
> >> driver no longer restarts when a CPU core is
> >> hot-disabled and then hot-enabled on AMD quad-core
> >> systems.
> >>
> >> The following comands work fine on 2.6.26 and fail
> >> on 2.6.27-rc1:
> >>
> >> echo 0 > /sys/devices/system/cpu/cpu3/online
> >> echo 1 > /sys/devices/system/cpu/cpu3/online
> >> find /sys -name cpufreq
> >>
> >> For 2.6.26, the find will return a cpufreq
> >> directory for each processor.  In 2.6.27-rc1,
> >> the cpu3 directory is missing.
> >>
> >> After digging through the code, the following
> >> logic is failing when the core is hot-enabled
> >> at runtime.  The code works during the boot
> >> sequence.
> >>
> >>       cpumask_t = current->cpus_allowed;
> >>       set_cpus_allowed_ptr(current, &cpumask_of_cpu(cpu));
> >>       if (smp_processor_id() != cpu)
> >>               return -ENODEV;
> >>
> 
> 
> if it gets called from any of the cpu-hotplug handlers, it won't work
> now (x86-microcode is another victim).
> 
> Please give a try to the following patch: http://lkml.org/lkml/2008/7/30/171
> 
> does it help?
> 
> (the explanation is also available in this thread).

i've queued up the fix below in tip/sched/urgent.

	Ingo

------------------------>
>From 3bc8fb8f85b79aa2d6341adb5090799b93d64bc9 Mon Sep 17 00:00:00 2001
From: Dmitry Adamushko <dmitry.adamushko@...il.com>
Date: Wed, 30 Jul 2008 12:34:04 +0200
Subject: [PATCH] sched, cpu hotplug: fix set_cpus_allowed() use in hotplug callbacks

Mark Langsdorf reported:

> One of my co-workers noticed that the powernow-k8
> driver no longer restarts when a CPU core is
> hot-disabled and then hot-enabled on AMD quad-core
> systems.
>
> The following comands work fine on 2.6.26 and fail
> on 2.6.27-rc1:
>
> echo 0 > /sys/devices/system/cpu/cpu3/online
> echo 1 > /sys/devices/system/cpu/cpu3/online
> find /sys -name cpufreq
>
> For 2.6.26, the find will return a cpufreq
> directory for each processor.  In 2.6.27-rc1,
> the cpu3 directory is missing.
>
> After digging through the code, the following
> logic is failing when the core is hot-enabled
> at runtime.  The code works during the boot
> sequence.
>
>       cpumask_t = current->cpus_allowed;
>       set_cpus_allowed_ptr(current, &cpumask_of_cpu(cpu));
>       if (smp_processor_id() != cpu)
>               return -ENODEV;

a similar problem also affects the microcode driver.

So set the CPU active before calling the CPU_ONLINE notifier chain,
there are a handful of notifiers that use set_cpus_allowed().

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 kernel/cpu.c |    5 ++---
 1 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index e202a68..c977c33 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -349,6 +349,8 @@ static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen)
 		goto out_notify;
 	BUG_ON(!cpu_online(cpu));
 
+	cpu_set(cpu, cpu_active_map);
+
 	/* Now call notifier in preparation. */
 	raw_notifier_call_chain(&cpu_chain, CPU_ONLINE | mod, hcpu);
 
@@ -383,9 +385,6 @@ int __cpuinit cpu_up(unsigned int cpu)
 
 	err = _cpu_up(cpu, 0);
 
-	if (cpu_online(cpu))
-		cpu_set(cpu, cpu_active_map);
-
 out:
 	cpu_maps_update_done();
 	return err;
--
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