linux-kernel - Re: [PATCH] pseries/hotplug: Add more delay in pseries_cpu

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Date:   Mon, 10 Dec 2018 18:31:43 -0200
From:   Thiago Jung Bauermann <bauerman@...ux.ibm.com>
To:     Michael Bringmann <mwb@...ux.vnet.ibm.com>
Cc:     ego@...ux.vnet.ibm.com, Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Tyrel Datwyler <tyreld@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pseries/hotplug: Add more delay in pseries_cpu_die while waiting for rtas-stop


Hello Michael,

Michael Bringmann <mwb@...ux.vnet.ibm.com> writes:

> I have asked Scott Mayes to take a look at one of these crashes from
> the phyp side.  I will let you know if he finds anything notable.

Thanks! It might make sense to test whether booting with
cede_offline=off makes the bug go away.

One suspicion I have is regarding the code handling CPU_STATE_INACTIVE.
>From what I understand, it is a powerpc-specific CPU state and from the
perspective of the generic CPU hotplug state machine, inactive CPUs are
already fully offline. Which means that the locking performed by the
generic code state machine doesn't apply to transitioning CPUs from
INACTIVE to OFFLINE state. Perhaps the bug is that there is more than
one CPU making that transition at the same time? That would cause two
CPUs to call RTAS stop-self.

I haven't checked whether this is really possible or not, though. It's
just a conjecture.

--
Thiago Jung Bauermann
IBM Linux Technology Center