On 8/29/2016 12:48 PM, Peter Zijlstra wrote:

On Mon, Aug 29, 2016 at 12:40:32PM -0400, Chris Metcalf wrote:

quoted

On 8/29/2016 12:33 PM, Peter Zijlstra wrote:

quoted

On Tue, Aug 16, 2016 at 05:19:27PM -0400, Chris Metcalf wrote:

quoted

+	/*
+	 * Request rescheduling unless we are in full dynticks mode.
+	 * We would eventually get pre-empted without this, and if
+	 * there's another task waiting, it would run; but by
+	 * explicitly requesting the reschedule, we may reduce the
+	 * latency.  We could directly call schedule() here as well,
+	 * but since our caller is the standard place where schedule()
+	 * is called, we defer to the caller.
+	 *
+	 * A more substantive approach here would be to use a struct
+	 * completion here explicitly, and complete it when we shut
+	 * down dynticks, but since we presumably have nothing better
+	 * to do on this core anyway, just spinning seems plausible.
+	 */
+	if (!tick_nohz_tick_stopped())
+		set_tsk_need_resched(current);

This is broken.. and it would be really good if you don't actually need
to do this.

Can you elaborate?

Naked use of TIF_NEED_RESCHED like this is busted. There is more state
that needs to be poked to keep things consistent / working.

Would it be cleaner to just replace the set_tsk_need_resched() call
with something like:

     set_current_state(TASK_INTERRUPTIBLE);
     schedule();
     __set_current_state(TASK_RUNNING);

or what would you recommend?

Or, as I said, just doing a busy loop here while testing to see
if need_resched or signal had been set?

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com

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