Date: Wed, 4 Jun 2014 15:21:55 +0200
From: Igor Mammedov <imammedo@redhat.com>
To: linux-kernel@vger.kernel.org
Cc: tglx@linutronix.de, mingo@redhat.com, x86@kernel.org
Subject: Re: [PATCH v5 0/4] x86: fix hang when AP bringup is too slow
Message-ID: <20140604152155.08e15821@nial.usersys.redhat.com>
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References: <1399322991-19329-1-git-send-email-imammedo@redhat.com>
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On Mon,  5 May 2014 22:49:47 +0200
Igor Mammedov <imammedo@redhat.com> wrote:

> changes since v4:
>  * merge "[PATCH v4 1/5] x86: fix list corruption on CPU hotplug"
>    and "[PATCH v4 2/5] x86: fix memory corruption in acpi_unmap_lsapic()"
>    together
>  * "x86: initialize secondary CPU only if master CPU will wait for it:
>     - add 10 seconds timeout description into commit message
>     - add smp_mb() after clearing cpu_initialized_mask
> 
> changes since v3:
>  * put simple bugfixes first
>  * move common part of syncing with master CPU in cpu_init()
>    for x32/64 variant into helper function
>  * cpu_init(): WARN_ON if cpu_initialized_mask is set
>  * fix panic on CPU unplug, caused by erroneous removing
>    of "pr->dev = dev;" in drivers/acpi/acpi_processor.c
> 
> --
> Hang is observed on virtual machines during CPU hotplug,
> especially in big guests with many CPUs. (It happens more
> often if host is over-committed).
> 
> Hang happens because master CPU timeouts on waiting till
> AP boots and 'cancels' CPU online operation assuming AP
> is not functional but AP may continue run wild later
> causing various hangs or panics in running kernel that
> is assuming that AP was offline.
> 
> This is an alternative approach, that instead of canceling
> in-progress AP bringup (https://lkml.org/lkml/2014/3/6/257),
> removes timeouts so that AP bringup won't be affected by
> poor timing and syncs AP with master CPU at early startup
> making sure that AP won't run wild if master CPU doesn't
> expect AP to come online.
> 
> Series also fixes 3 bugs found during testing CPU bringup
> failure case.

since 3.16 merge window is open now,
ping

> --
> Below is the detailed description of a more often happening hang:
> ---
> Master CPU may timeout before cpu_callin_mask is set and cancel
> booting CPU, but being onlined CPU still continues to boot, sets
> cpu_active_mask (CPU_STARTING notifiers) and spins in
> check_tsc_sync_target() for master cpu to arrive. Following attempt
> to online another cpu hangs in stop_machine, initiated from here:
> smp_callin ->
>   smp_store_cpu_info ->
>     identify_secondary_cpu ->
>       mtrr_ap_init -> set_mtrr_from_inactive_cpu
> 
> stop_machine waits on completion of stop_work on all CPUs from
> cpu_active_mask including a failed CPU that spins in check_tsc_sync_target().
> 
> Igor Mammedov (4):
>   x86: fix list/memory corruption on CPU hotplug
>   acpi_processor: do not mark present at boot but not onlined CPU as
>     onlined
>   x86: log error on secondary CPU wakeup failure at ERR level
>   x86: initialize secondary CPU only if master CPU will wait for it
> 
>  arch/x86/kernel/cpu/common.c  |   27 ++++++----
>  arch/x86/kernel/smpboot.c     |  104 +++++++++++++----------------------------
>  drivers/acpi/acpi_processor.c |    1 -
>  3 files changed, 48 insertions(+), 84 deletions(-)
> 

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