We have been observing hangs, both in KVM guest vcpus and more
generally, where a process that is woken doesn't properly wake up and
continue to run, but instead sticks in TASK_WAKING state. This
happens because the update of rq->wake_list in ttwu_queue_remote()
is not ordered with the update of ipi_message in
smp_muxed_ipi_message_pass(), and the reading of rq->wake_list in
scheduler_ipi() is not ordered with the reading of ipi_message in
smp_ipi_demux(). Thus it is possible for the IPI receiver not to see
the updated rq->wake_list and therefore conclude that there is nothing
for it to do.
In order to make sure that anything done before calling
smp_send_reschedule() is ordered before anything done in the resulting
call to scheduler_ipi(), this adds barriers in
smp_muxed_message_pass() and smp_ipi_demux(). The barrier in
smp_muxed_message_pass() is a full barrier to ensure that there is a
full ordering between the smp_send_reschedule() caller and
scheduler_ipi(). In smp_ipi_demux(), we use xchg() rather than
xchg_local() because xchg() includes release and acquire barriers.
Using xchg() rather than xchg_local() makes sense given that
ipi_message is not just accessed locally.
These changes made no measurable difference to the speed of IPIs as
measured using a simple ping-pong latency test across two CPUs on
different cores of a POWER7 machine.
The analysis of the reason why processes were not waking up properly
is due to Milton Miller.
Cc: sta...@vger.kernel.org
Reported-by: Milton Miller <milt...@bga.com>
Signed-off-by: Paul Mackerras <pau...@samba.org>
---
arch/powerpc/kernel/smp.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index e292ff2..ca1040a 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -197,8 +197,15 @@ void smp_muxed_ipi_message_pass(int cpu, int msg)
struct cpu_messages *info = &per_cpu(ipi_message, cpu);
char *message = (char *)&info->messages;
+ /*
+ * Order previous accesses before accesses in the IPI handler.
+ */
+ smp_mb();
message[msg] = 1;
- mb();
+ /*
+ * Order setting of message before IPI.
+ */
+ smp_wmb();
smp_ops->cause_ipi(cpu, info->data);
}
@@ -210,7 +217,7 @@ irqreturn_t smp_ipi_demux(void)
mb(); /* order any irq clear */
do {
- all = xchg_local(&info->messages, 0);
+ all = xchg(&info->messages, 0);
#ifdef __BIG_ENDIAN
if (all & (1 << (24 - 8 * PPC_MSG_CALL_FUNCTION)))
--
1.7.10.rc3.219.g53414
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