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Date: Tue, 12 Mar 2019 15:02:46 +0530
From: Gautham R Shenoy <ego@linux.vnet.ibm.com>
To: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Subject: Re: [PATCH v3] powerpc/pseries: Only wait for dying CPU after call
 to rtas_stop_self()
References: <20190311193517.23756-1-bauerman@linux.ibm.com>
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Reply-To: ego@linux.vnet.ibm.com
Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com>, linux-kernel@vger.kernel.org,
 Michael Bringmann <mwb@linux.vnet.ibm.com>,
 Tyrel Datwyler <tyreld@linux.vnet.ibm.com>, linuxppc-dev@lists.ozlabs.org
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Hello Thiago,

On Mon, Mar 11, 2019 at 04:35:17PM -0300, Thiago Jung Bauermann wrote:
> When testing DLPAR CPU add/remove on a system under stress,
> pseries_cpu_die() doesn't wait long enough for a CPU to die:
> 
> [  446.983944] cpu 148 (hwid 148) Ready to die...
> [  446.984062] cpu 149 (hwid 149) Ready to die...
> [  446.993518] cpu 150 (hwid 150) Ready to die...
> [  446.993543] Querying DEAD? cpu 150 (150) shows 2
> [  446.994098] cpu 151 (hwid 151) Ready to die...
> [  447.133726] cpu 136 (hwid 136) Ready to die...
> [  447.403532] cpu 137 (hwid 137) Ready to die...
> [  447.403772] cpu 138 (hwid 138) Ready to die...
> [  447.403839] cpu 139 (hwid 139) Ready to die...
> [  447.403887] cpu 140 (hwid 140) Ready to die...
> [  447.403937] cpu 141 (hwid 141) Ready to die...
> [  447.403979] cpu 142 (hwid 142) Ready to die...
> [  447.404038] cpu 143 (hwid 143) Ready to die...
> [  447.513546] cpu 128 (hwid 128) Ready to die...
> [  447.693533] cpu 129 (hwid 129) Ready to die...
> [  447.693999] cpu 130 (hwid 130) Ready to die...
> [  447.703530] cpu 131 (hwid 131) Ready to die...
> [  447.704087] Querying DEAD? cpu 132 (132) shows 2
> [  447.704102] cpu 132 (hwid 132) Ready to die...
> [  447.713534] cpu 133 (hwid 133) Ready to die...
> [  447.714064] Querying DEAD? cpu 134 (134) shows 2
> 
> This is a race between one CPU stopping and another one calling
> pseries_cpu_die() to wait for it to stop. That function does a short busy
> loop calling RTAS query-cpu-stopped-state on the stopping CPU to verify
> that it is stopped, but I think there's a lot for the stopping CPU to do
> which may take longer than this loop allows.
> 
> As can be seen in the dmesg right before or after the "Querying DEAD?"
> messages, if pseries_cpu_die() waited a little longer it would have seen
> the CPU in the stopped state.
> 
> What I think is going on is that CPU 134 was inactive at the time it was
> unplugged. In that case, dlpar_offline_cpu() calls H_PROD on that CPU and
> immediately calls pseries_cpu_die(). Meanwhile, the prodded CPU activates
> and start the process of stopping itself. The busy loop is not long enough
> to allow for the CPU to wake up and complete the stopping process.
> 
> This can be a problem because if the busy loop finishes too early, then the
> kernel may offline another CPU before the previous one finished dying,
> which would lead to two concurrent calls to rtas-stop-self, which is
> prohibited by the PAPR.
> 
> We can make the race a lot more even if we only start querying if the CPU
> is stopped when the stopping CPU is close to call rtas_stop_self(). Since
> pseries_mach_cpu_die() sets the CPU current state to offline almost
> immediately before calling rtas_stop_self(), we use that as a signal that
> it is either already stopped or very close to that point, and we can start
> the busy loop.
> 
> As suggested by Michael Ellerman, this patch also changes the busy loop to
> wait for a fixed amount of wall time. Based on the measurements that
> Gautham did on a POWER9 system, in successful cases of
> smp_query_cpu_stopped(cpu) returning affirmative, the maximum time spent
> inside the loop was was 10 ms. This patch loops for 20 ms just be sure.
> 
> Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>

Thanks for this version. I have tested the patch and we no longer see
the "Querying DEAD? cpu X (Y) shows 2" message.


Tested-and-Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>


--
Thanks and Regards
gautham.