On Wed, Aug 01, 2018 at 11:02:59PM +1000, Michael Ellerman wrote:
Hi John,
I'm still not sure about this one.
John Allen <jal...@linux.ibm.com> writes:
On Mon, Jul 23, 2018 at 11:27:56PM +1000, Michael Ellerman wrote:
Hi John,
I'm a bit puzzled by this one.
John Allen <jal...@linux.ibm.com> writes:
When a PRRN event is being handled and another PRRN event comes in, the
second event will block rtas polling waiting on the first to complete,
preventing any further rtas events from being handled. This can be
especially problematic in case that PRRN events are continuously being
queued in which case rtas polling gets indefinitely blocked completely.
This patch introduces a mutex that prevents any subsequent PRRN events from
running while there is a prrn event being handled, allowing rtas polling to
continue normally.
Signed-off-by: John Allen <jal...@linux.ibm.com>
---
v2:
-Unlock prrn_lock when PRRN operations are complete, not after handler is
scheduled.
-Remove call to flush_work, the previous broken method of serializing
PRRN events.
---
arch/powerpc/kernel/rtasd.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)
diff --git a/arch/powerpc/kernel/rtasd.c b/arch/powerpc/kernel/rtasd.c
index 44d66c33d59d..845fc5aec178 100644
--- a/arch/powerpc/kernel/rtasd.c
+++ b/arch/powerpc/kernel/rtasd.c
@@ -284,15 +286,17 @@ static void prrn_work_fn(struct work_struct *work)
*/
pseries_devicetree_update(-prrn_update_scope);
numa_update_cpu_topology(false);
+ mutex_unlock(&prrn_lock);
}
static DECLARE_WORK(prrn_work, prrn_work_fn);
static void prrn_schedule_update(u32 scope)
{
- flush_work(&prrn_work);
This seems like it's actually the core of the change. Previously we were
basically blocking on the flush before continuing.
The idea here is to replace the blocking flush_work with a non-blocking
mutex. So rather than waiting on the running PRRN event to complete, we
bail out since a PRRN event is already running.
OK, but why is it OK to bail out?
The firmware sent you an error log asking you to do something, with a
scope value that has some meaning, and now you're just going to drop
that on the floor?
Maybe it is OK to just drop these events? Or maybe you're saying that
because the system is crashing under the load of too many events it's OK
to drop the events in this case.
I think I see your point. If a PRRN event comes in while another is
currently running, the new one may contain a different list of LMBs/CPUs
and the old list becomes outdated. With the mutex, the only event that
gets handled is the oldest and we will lose any additional changes
beyond the initial event. Therefore, as you mentioned in your previous
message, the behavior of the global workqueue should work just fine once
we remove the call to flush_work. While a prrn event is running, only
one will remain on the workqueue, then when the first one completes, the
newly scheduled work function should grab the latest PRRN list.
I will send a new version of the patch with just the call to flush_work
removed.
-John
The situation this is
meant to address is flooding the workqueue with PRRN events, which like
the situation in patch 2/2, these can be queued up faster than they can
actually be handled.
I'm not really sure why this is a problem though.
The current code synchronously processes the events, so there should
only ever be one in flight.
I guess the issue is that each one can queue multiple events on the
hotplug work queue?
But still, we have terabytes of RAM, we should be able to queue a lot
of events before it becomes a problem.
So what exactly is getting flooded, what's the symptom?
If the queuing of the hotplug events is the problem, then why don't we
stop doing that? We could just process them synchronously from the PRRN
update, that would naturally throttle them.
cheers
