Re: [BUGFIX][PATCH] Freezer, CPU hotplug, x86 Microcode: Fix task freezing failures

From: Rafael J. Wysocki
Date: Sun Oct 02 2011 - 15:34:28 EST


Thanks for the fix.

On Sunday, October 02, 2011, Srivatsa S. Bhat wrote:
> This patch addresses the warnings found in the logs in the
> task freezing failure bug reported in
> The warnings appear because of the reason explained below:
> There are microcode callbacks registered for CPU hotplug events such
> as a CPU getting offlined or onlined. When a CPU is offlined
> with tasks being frozen (as in the case of disabling the non-boot CPUs
> while preparing for a system suspend operation), the CPU_DEAD_FROZEN
> notification is sent, for which the microcode callback does not
> do anything. In particular, it does not free or invalidate the CPU
> microcode which it had got from userspace earlier. Hence when that CPU
> comes back online with tasks being frozen (as in the case of re-enabling
> the non-boot CPUs during a resume operation after suspend), the microcode
> callback applies the microcode (which it already possesses) to that CPU.
> However, during a pure CPU hotplug operation, tasks are not frozen and
> hence the CPU_DEAD notification is sent. Upon this event notification,
> the microcode callback frees the copy of microcode it has and
> invalidates it. And during a CPU online, it tries to apply the microcode
> to the CPU, but since it doesn't have the copy of the microcode, it depends
> on a userspace utility to get the microcode. This is perfectly fine when
> doing plain CPU hotplug operations alone.
> Things go wrong when a CPU hotplug stress test is carried out along with
> a suspend/resume operation running simultaneously. Upon getting a CPU_DEAD
> notification (for example, when a CPU offline occurs with tasks not frozen),
> the microcode callback frees up the microcode and invalidates it. Later
> when that CPU gets onlined with tasks being frozen, the microcode callback
> (for the CPU_ONLINE_FROZEN event) tries to apply the microcode to the CPU;
> doesn't find it and hence depends on the (currently frozen) userspace to
> get the microcode again. This leads to the numerous "WARNING"s at
> drivers/base/firmware_class.c which eventually leads to task freezing failures
> in the suspend code path, as has been reported.
> So, this patch addresses this issue by ensuring that microcode is not freed
> from kernel memory, nor invalidated when a CPU goes offline. Thus once the
> kernel gets the microcode during boot-up, it will never have to depend on
> userspace ever again to get microcode, since it never releases the copy it
> already has. So every run of the microcode callback for CPU online event will
> now succeed irrespective of whether userspace is frozen or not. As a result,
> this fixes the task freezing failure encountered while running CPU hotplug
> stress test along with suspend/resume operations simultaneously.
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> ---

Thanks for the fix. I'd like to push it for 3.2 and possibly -stable.

Does anyone have any objections?


> arch/x86/kernel/microcode_core.c | 10 +++++++++-
> 1 files changed, 9 insertions(+), 1 deletions(-)
> diff --git a/arch/x86/kernel/microcode_core.c b/arch/x86/kernel/microcode_core.c
> index f924280..cd7ef2f 100644
> --- a/arch/x86/kernel/microcode_core.c
> +++ b/arch/x86/kernel/microcode_core.c
> @@ -483,7 +483,15 @@ mc_cpu_callback(struct notifier_block *nb, unsigned long action, void *hcpu)
> sysfs_remove_group(&sys_dev->kobj, &mc_attr_group);
> pr_debug("CPU%d removed\n", cpu);
> break;
> - case CPU_DEAD:
> +
> + /*
> + * Do not invalidate the microcode if a CPU goes offline,
> + * because it would be impossible to get the microcode again
> + * from userspace when the CPU comes back up, if the userspace
> + * happens to be frozen at that moment by the freezer subsystem,
> + * for example, due to a suspend operation in progress.
> + */
> +
> /* The CPU refused to come up during a system resume */
> microcode_fini_cpu(cpu);

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