On Thu, Oct 05, 2017 at 06:16:23PM +0200, Thomas Gleixner wrote:

On Thu, 5 Oct 2017, Julia Cartwright wrote:

quoted

On Thu, Oct 05, 2017 at 11:55:39AM -0400, Steven Rostedt wrote:

quoted

On Thu, 5 Oct 2017 10:37:59 -0500
Julia Cartwright [off-list ref] wrote:

quoted

On Thu, Oct 05, 2017 at 05:27:30PM +0200, Thomas Gleixner wrote:

quoted

On Thu, 5 Oct 2017, Julia Cartwright wrote:  

quoted

On Tue, Oct 03, 2017 at 12:49:19PM -0300, Arnaldo Carvalho de Melo wrote:  

quoted

-	preempt_disable();
+	preempt_disable_nort();
 	this_cpu_inc(*sc->buffers_allocated);  

Have you tried this on RT w/ CONFIG_DEBUG_PREEMPT?  I believe that the
this_cpu_* operations perform a preemption check, which we'd trip.  

Good point. Changing this to migrate_disable() would do the trick.  

Wouldn't we still trip the preempt check even with migration disabled?
In another thread I asked the same question: should the preemption
checks here be converted to migration-checks in RT?

Is it a "preemption check"?

Sorry if I was unclear, more precisely: the this_cpu_* family of
accessors, w/ CONFIG_DEBUG_PREEMPT currently spits out a warning when
the caller is invoked in a context where preemption is enabled.

The check is shared w/ the smp_processor_id() check, as implemented in
lib/smp_processor_id.c.  It effectively boils down to a check of
preempt_count() and irqs_disabled().

Except that on RT that check cares about the migrate disable state. You can
invoke this_cpu_* and smp_processor_id() in preemptible/interruptible
context because of:

	if (cpumask_equal(current->cpus_ptr, cpumask_of(this_cpu)))
		goto out;

That's true even on mainline.

But you are right that this check needs some update because
migrate_disable() does not immediately update the allowed cpumask IIRC.

Actually, I think it does:

   migrate_disable() ->
      p = current;
      ...
      migrate_disable_update_cpus_allowed(p) ->
          p->cpus_ptr = cpumask_of(smp_processor_id())
      ...

Perhaps it's worth a simple comment update, below.

   Julia

-- 8< --
Subject: [PATCH] kernel: sched: document smp_processor_id/this_cpu safety in
 migration-disabled context

On RT, users of smp_processor_id() and this_cpu_*() per-cpu accessors
are considered safe if the caller executes with migration disabled.  On
!RT, preempt_disable() is sufficient to make this guarantee, however on
RT, the lesser migrate_disable() is sufficient.

It is not entirely obvious which check in check_preemption_disabled()
makes this work, so document it.

Signed-off-by: Julia Cartwright <redacted>
---
 lib/smp_processor_id.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/lib/smp_processor_id.c b/lib/smp_processor_id.c
index de3b2d925473..c8091d9eb1b4 100644
--- a/lib/smp_processor_id.c
+++ b/lib/smp_processor_id.c

@@ -20,7 +20,11 @@ notrace static unsigned int check_preemption_disabled(const char *what1,
 
 	/*
 	 * Kernel threads bound to a single CPU can safely use
-	 * smp_processor_id():
+	 * smp_processor_id().
+	 *
+	 * In addition, threads which are currently executing within
+	 * a migration disabled region can safely use smp_processor_id() and
+	 * this_cpu accessors.
 	 */
 	if (cpumask_equal(current->cpus_ptr, cpumask_of(this_cpu)))
 		goto out;

-- 
2.14.1