freezer: skip waking up tasks with PF_FREEZER_SKIP set

Android goes through suspend/resume very often (every few seconds when
on a busy wifi network with the screen off), and a significant portion
of the energy used to go in and out of suspend is spent in the
freezer.  If a task has called freezer_do_not_count(), don't bother
waking it up.  If it happens to wake up later it will call
freezer_count() and immediately enter the refrigerator.

Combined with patches to convert freezable helpers to use
freezer_do_not_count() and convert common sites where idle userspace
tasks are blocked to use the freezable helpers, this reduces the
time and energy required to suspend and resume.

Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/kernel/freezer.c b/kernel/freezer.c
index c38893b0efba..8b2afc1c9df0 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -110,6 +110,18 @@ bool freeze_task(struct task_struct *p)
 {
         unsigned long flags;
 
+        /*
+         * This check can race with freezer_do_not_count, but worst case that
+         * will result in an extra wakeup being sent to the task.  It does not
+         * race with freezer_count(), the barriers in freezer_count() and
+         * freezer_should_skip() ensure that either freezer_count() sees
+         * freezing == true in try_to_freeze() and freezes, or
+         * freezer_should_skip() sees !PF_FREEZE_SKIP and freezes the task
+         * normally.
+         */
+        if (freezer_should_skip(p))
+                return false;
+
         spin_lock_irqsave(&freezer_lock, flags);
         if (!freezing(p) || frozen(p)) {
                 spin_unlock_irqrestore(&freezer_lock, flags);