On 08/24/2010 12:24 AM, Benjamin Herrenschmidt wrote:

On Wed, 2010-08-11 at 15:34 -0500, Brian King wrote:

quoted

While testing CPU DLPAR, the following problem was discovered.
We were DLPAR removing the first CPU, which in this case was
logical CPUs 0-3. CPUs 0-2 were already marked offline and
we were in the process of offlining CPU 3. After marking
the CPU inactive and offline in cpu_disable, but before the
cpu was completely idle (cpu_die), we ended up in __make_request
on CPU 3. There we looked at the topology map to see which CPU
to complete the I/O on and found no CPUs in the cpu_sibling_map.
This resulted in the block layer setting the completion cpu
to be NR_CPUS, which then caused an oops when we tried to
complete the I/O.

Fix this by delaying clearing the sibling map of the cpu we
are offlining for the cpu we are offlining until cpu_die.

So I'm not getting a clear mental picture of the situation, sorry about
that.

We are offlining CPU 3, and we have already marked it inactive and
online, so how come we end up in __make_request() on it at this stage

I'm not sure about that. My thought was that until we get into cpu_die,
the cpu could still be executing code.

and shouldn't it be the block layer that notices that it's targeting an
offlined CPU ?

It could be easily fixed in blk_cpu_to_group as well. I'll look into
this.

IE. I have doubts about leaving a CPU in the sibling map which isn't
online... Wouldn't we end up "scheduling" things to it after it's
supposed to have freed itself of everything (timers, workqueues,
etc...) ?

I was assuming this wouldn't happen since the cpu is no longer online.


Thanks,

Brian

As I said, I'm probably missing a part of the puzzle ..

Ben.

quoted

Signed-off-by: Brian King <redacted>
---

 arch/powerpc/kernel/smp.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff -puN arch/powerpc/kernel/smp.c~powerpc_sibling_map_offline arch/powerpc/kernel/smp.c

--- linux-2.6/arch/powerpc/kernel/smp.c~powerpc_sibling_map_offline	2010-08-09 16:49:47.000000000 -0500
+++ linux-2.6-bjking1/arch/powerpc/kernel/smp.c	2010-08-09 16:49:47.000000000 -0500

@@ -598,8 +598,11 @@ int __cpu_disable(void)
 	/* Update sibling maps */
 	base = cpu_first_thread_in_core(cpu);
 	for (i = 0; i < threads_per_core; i++) {
-		cpumask_clear_cpu(cpu, cpu_sibling_mask(base + i));
-		cpumask_clear_cpu(base + i, cpu_sibling_mask(cpu));
+		if ((base + i) != cpu) {
+			cpumask_clear_cpu(cpu, cpu_sibling_mask(base + i));
+			cpumask_clear_cpu(base + i, cpu_sibling_mask(cpu));
+		}
+
 		cpumask_clear_cpu(cpu, cpu_core_mask(base + i));
 		cpumask_clear_cpu(base + i, cpu_core_mask(cpu));
 	}

@@ -641,6 +644,8 @@ void cpu_hotplug_driver_unlock()
 
 void cpu_die(void)
 {
+	cpumask_clear_cpu(smp_processor_id(), cpu_sibling_mask(smp_processor_id()));
+
 	if (ppc_md.cpu_die)
 		ppc_md.cpu_die();
 }

_

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-- 
Brian King
Linux on Power Virtualization
IBM Linux Technology Center