[PATCH v3 2/2] ksm: provide support to use deferrable timers for scanner thread

[Chintan Pandya <cpandya@codeaurora.org>]

KSM thread to scan pages is scheduled on definite timeout.  That wakes up
CPU from idle state and hence may affect the power consumption.  Provide
an optional support to use deferrable timer which suites low-power
use-cases.

Typically, on our setup we observed, 10% less power consumption with some
use-cases in which CPU goes to power collapse frequently.  For example,
playing audio while typically CPU remains idle.

To enable deferrable timers,
$ echo 1 > /sys/kernel/mm/ksm/deferrable_timer

Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
---
Changes:

V2-->V3:
	- Handled error case properly
	- Corrected indentation in Documentation
	- Fixed build failure
	- Removed left over process_timeout()
V1-->V2:
	- allowing only valid values to be updated as use_deferrable_timer
	- using only 'deferrable' and not 'deferred'
	- moved out schedule_timeout code for deferrable timer into timer.c

 Documentation/vm/ksm.txt |  7 +++++++
 mm/ksm.c                 | 36 ++++++++++++++++++++++++++++++++++--
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/Documentation/vm/ksm.txt b/Documentation/vm/ksm.txt
index f34a8ee..9735c87 100644
--- a/Documentation/vm/ksm.txt
+++ b/Documentation/vm/ksm.txt
@@ -87,6 +87,13 @@ pages_sharing    - how many more sites are sharing them i.e. how much saved
 pages_unshared   - how many pages unique but repeatedly checked for merging
 pages_volatile   - how many pages changing too fast to be placed in a tree
 full_scans       - how many times all mergeable areas have been scanned
+deferrable_timer - whether to use deferrable timers or not
+                   e.g. "echo 1 > /sys/kernel/mm/ksm/deferrable_timer"
+                   Default: 0 (means, we are not using deferrable timers. Users
+		   might want to set deferrable_timer option if they donot want
+		   ksm thread to wakeup CPU to carryout ksm activities thus
+		   gaining on battery while compromising slightly on memory
+		   that could have been saved.)
 
 A high ratio of pages_sharing to pages_shared indicates good sharing, but
 a high ratio of pages_unshared to pages_sharing indicates wasted effort.
diff --git a/mm/ksm.c b/mm/ksm.c
index fb75902..434a50a 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -223,6 +223,9 @@ static unsigned int ksm_thread_pages_to_scan = 100;
 /* Milliseconds ksmd should sleep between batches */
 static unsigned int ksm_thread_sleep_millisecs = 20;
 
+/* Boolean to indicate whether to use deferrable timer or not */
+static bool use_deferrable_timer;
+
 #ifdef CONFIG_NUMA
 /* Zeroed when merging across nodes is not allowed */
 static unsigned int ksm_merge_across_nodes = 1;
@@ -1725,8 +1728,13 @@ static int ksm_scan_thread(void *nothing)
 		try_to_freeze();
 
 		if (ksmd_should_run()) {
-			schedule_timeout_interruptible(
-				msecs_to_jiffies(ksm_thread_sleep_millisecs));
+			signed long to;
+
+			to = msecs_to_jiffies(ksm_thread_sleep_millisecs);
+			if (use_deferrable_timer)
+				schedule_timeout_deferrable_interruptible(to);
+			else
+				schedule_timeout_interruptible(to);
 		} else {
 			wait_event_freezable(ksm_thread_wait,
 				ksmd_should_run() || kthread_should_stop());
@@ -2175,6 +2183,29 @@ static ssize_t run_store(struct kobject *kobj, struct kobj_attribute *attr,
 }
 KSM_ATTR(run);
 
+static ssize_t deferrable_timer_show(struct kobject *kobj,
+				    struct kobj_attribute *attr, char *buf)
+{
+	return snprintf(buf, 8, "%d\n", use_deferrable_timer);
+}
+
+static ssize_t deferrable_timer_store(struct kobject *kobj,
+				     struct kobj_attribute *attr,
+				     const char *buf, size_t count)
+{
+	unsigned long enable;
+	int err;
+
+	err = kstrtoul(buf, 10, &enable);
+	if (err < 0)
+		return err;
+	if (enable >= 1)
+		return -EINVAL;
+	use_deferrable_timer = enable;
+	return count;
+}
+KSM_ATTR(deferrable_timer);
+
 #ifdef CONFIG_NUMA
 static ssize_t merge_across_nodes_show(struct kobject *kobj,
 				struct kobj_attribute *attr, char *buf)
@@ -2287,6 +2318,7 @@ static struct attribute *ksm_attrs[] = {
 	&pages_unshared_attr.attr,
 	&pages_volatile_attr.attr,
 	&full_scans_attr.attr,
+	&deferrable_timer_attr.attr,
 #ifdef CONFIG_NUMA
 	&merge_across_nodes_attr.attr,
 #endif
-- 
Chintan Pandya

QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
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