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    SubjectRe: Regression in 2.6.27-rc1 for set_cpus_allowed_ptr
    2008/8/11 Ingo Molnar <mingo@elte.hu>:
    >
    > * Dmitry Adamushko <dmitry.adamushko@gmail.com> wrote:
    >
    >> 2008/8/8 Rafael J. Wysocki <rjw@sisk.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@gmail.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@elte.hu>

    Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.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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