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

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Date:	Mon, 11 Aug 2008 16:28:11 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
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

2008/8/11 Ingo Molnar <mingo@...e.hu>:
>
> * 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>

Signed-off-by: Dmitry Adamushko <dmitry.adamushko@...il.com>

This fix also solves the problem with x86-microcode. I've sent
alternative patches for microcode, but as this "rely on
set_cpus_allowed_ptr() being workable in cpu-hotplug(CPU_ONLINE, ...)"
thing seems to be more broad than what we thought, perhaps this fix
should be applied.

With this patch we define that by the moment CPU_ONLINE is being sent,
a 'cpu' is online and ready for tasks to be migrated onto it.


> ---
>  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;
>



-- 
Best regards,
Dmitry Adamushko
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