On Tue, Aug 09, 2016 at 04:29:46PM -0400, Chris Metcalf wrote:

+/*
+ * Each time we try to prepare for return to userspace in a process
+ * with task isolation enabled, we run this code to quiesce whatever
+ * subsystems we can readily quiesce to avoid later interrupts.
+ */
+void task_isolation_enter(void)
+{
+	WARN_ON_ONCE(irqs_disabled());
+
+	/* Drain the pagevecs to avoid unnecessary IPI flushes later. */
+	lru_add_drain();
+
+	/* Quieten the vmstat worker so it won't interrupt us. */
+	quiet_vmstat_sync();

So, this is going to be called everytime we resume to userspace
while in task isolation mode, right?

Do we need to quiesce vmstat everytime before entering userspace?
I thought that vmstat only need to be offlined once and for all?

And how about lru?

+
+	/*
+	 * 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);

Again, that won't help :-)

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