linux-kernel - Re: [PATCH V4] tick/hotplug: Handover time related duties before cpu offline

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Date:	Tue, 17 Feb 2015 12:58:04 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, aik@...abs.ru, shreyas@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, anton@...ba.org,
	svaidy@...ux.vnet.ibm.com, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH V4] tick/hotplug: Handover time related duties before
 cpu offline

On Sat, 2015-01-31 at 09:44 +0530, Preeti U Murthy wrote:
> These duties include do_timer to update jiffies and broadcast wakeups on those
> platforms which do not have an external device to handle wakeup of cpus from deep
> idle states. The handover of these duties is not robust against a cpu offline
> operation today.
> 
> The do_timer duty is handed over in the CPU_DYING phase today to one of the online
> cpus. This relies on the fact that *all* cpus participate in stop_machine phase.
> But if this design is to change in the future, i.e. if all cpus are not
> required to participate in stop_machine, the freshly nominated do_timer cpu
> could be idle at the time of handover. In that case, unless its interrupted,
> it will not wakeup to update jiffies and timekeeping will hang.
> 
> With regard to broadcast wakeups, today if the cpu handling broadcast of wakeups
> goes offline, the job of broadcasting is handed over to another cpu in the CPU_DEAD
> phase. The CPU_DEAD notifiers are run only after the offline cpu sets its state as
> CPU_DEAD. Meanwhile, the kthread doing the offline is scheduled out while waiting for
> this transition by queuing a timer. This is fatal because if the cpu on which
> this kthread was running has no other work queued on it, it can re-enter deep
> idle state, since it sees that a broadcast cpu still exists. However the broadcast
> wakeup will never come since the cpu which was handling it is offline, and the cpu
> on which the kthread doing the hotplug operation was running never wakes up to see
> this because its in deep idle state.
> 
> Fix these issues by handing over the do_timer and broadcast wakeup duties just before
> the offline cpu kills itself, to the cpu performing the hotplug operation. Since the
> cpu performing the hotplug operation is up and running, it becomes aware of the handover
> of do_timer duty and queues the broadcast timer upon itself so as to seamlessly
> continue both these operations.
> 
> It fixes the bug reported here:
> http://linuxppc.10917.n7.nabble.com/offlining-cpus-breakage-td88619.html
> 
> Signed-off-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> ---
> Changes from V3: https://lkml.org/lkml/2015/1/20/236
> 1. Move handover of broadcast duty away from CPU_DYING phase to just before
> the cpu kills itself.
> 2. Club the handover of timekeeping duty along with broadcast duty to make
> timekeeping robust against hotplug.

Hi Preeti,

This bug is still causing breakage for people on Power8 machines.

Are we just waiting for Thomas to take the patch?

cheers


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