[PATCH v3] suspend: make sync() on suspend-to-RAM optional

[Len Brown]

From: Len Brown <len.brown@xxxxxxxxx>

Linux suspend-to-RAM was unreliable when first developed,
and so sys_sync() was invoked inside the kernel at the
start of every suspend flow.

Today, many devices invoke suspend with
high reliability and high frequency.
They may not want to be forced to pay
the performance cost of sync() on every suspend.

So here we make the file system sync() optional.
De-select CONFIG_SUSPEND_FS_SYNC to delete the call.
Keep CONFIG_SUSPEND_FS_SYNC keept the call, and also
includes the new sysfs attribte,
/sys/power/suspend_fs_sync
which can be cleared to disable the file-system sync.

As we have had this call for a long time,
the default remains to keep the call, and to
invoke it on every suspend.

Signed-off-by: Len Brown <len.brown@xxxxxxxxx>
---
 kernel/power/Kconfig   | 11 +++++++++++
 kernel/power/main.c    | 33 +++++++++++++++++++++++++++++++++
 kernel/power/power.h   |  1 +
 kernel/power/suspend.c | 10 +++++++---
 4 files changed, 52 insertions(+), 3 deletions(-)

diff --git a/kernel/power/Kconfig b/kernel/power/Kconfig
index 2fac9cc..244c2d9 100644
--- a/kernel/power/Kconfig
+++ b/kernel/power/Kconfig
@@ -102,6 +102,17 @@ config PM_SLEEP_SMP
 	depends on PM_SLEEP
 	select HOTPLUG_CPU
 
+config SUSPEND_FS_SYNC
+	bool "Suspend to RAM includes file-system sync()"
+	default y
+	depends on SUSPEND
+	---help---
+	Include a file system sync() at the start of every suspend to RAM flow.
+	Disable this option to exclude that capability.
+
+	If included, the sync can still be skipped
+	by clearing /sys/power/suspend_fs_sync.
+
 config PM_AUTOSLEEP
 	bool "Opportunistic sleep"
 	depends on PM_SLEEP
diff --git a/kernel/power/main.c b/kernel/power/main.c
index 1d1bf63..bce7807 100644
--- a/kernel/power/main.c
+++ b/kernel/power/main.c
@@ -71,6 +71,36 @@ static ssize_t pm_async_store(struct kobject *kobj, struct kobj_attribute *attr,
 
 power_attr(pm_async);
 
+#ifdef CONFIG_SUSPEND_FS_SYNC
+
+/* If set, sync file systems at start of suspend flow */
+int suspend_fs_sync_enabled = 1;
+
+static ssize_t suspend_fs_sync_show(struct kobject *kobj, struct kobj_attribute *attr,
+			     char *buf)
+{
+	return sprintf(buf, "%d\n", suspend_fs_sync_enabled);
+}
+
+static ssize_t suspend_fs_sync_store(struct kobject *kobj, struct kobj_attribute *attr,
+			      const char *buf, size_t n)
+{
+	unsigned long val;
+
+	if (kstrtoul(buf, 10, &val))
+		return -EINVAL;
+
+	if (val > 1)
+		return -EINVAL;
+
+	suspend_fs_sync_enabled = val;
+	return n;
+}
+
+power_attr(suspend_fs_sync);
+
+#endif /* CONFIG_SUSPEND_FS_SYNC */
+
 #ifdef CONFIG_PM_DEBUG
 int pm_test_level = TEST_NONE;
 
@@ -592,6 +622,9 @@ static struct attribute * g[] = {
 #ifdef CONFIG_PM_SLEEP
 	&pm_async_attr.attr,
 	&wakeup_count_attr.attr,
+#ifdef CONFIG_SUSPEND_FS_SYNC
+	&suspend_fs_sync_attr.attr,
+#endif
 #ifdef CONFIG_PM_AUTOSLEEP
 	&autosleep_attr.attr,
 #endif
diff --git a/kernel/power/power.h b/kernel/power/power.h
index 7d4b7ff..3d86f09 100644
--- a/kernel/power/power.h
+++ b/kernel/power/power.h
@@ -200,6 +200,7 @@ static inline void suspend_test_finish(const char *label) {}
 #ifdef CONFIG_PM_SLEEP
 /* kernel/power/main.c */
 extern int pm_notifier_call_chain(unsigned long val);
+extern int suspend_fs_sync_enabled;
 #endif
 
 #ifdef CONFIG_HIGHMEM
diff --git a/kernel/power/suspend.c b/kernel/power/suspend.c
index 62ee437..e7eb2ba 100644
--- a/kernel/power/suspend.c
+++ b/kernel/power/suspend.c
@@ -333,9 +333,13 @@ static int enter_state(suspend_state_t state)
 	if (state == PM_SUSPEND_FREEZE)
 		freeze_begin();
 
-	printk(KERN_INFO "PM: Syncing filesystems ... ");
-	sys_sync();
-	printk("done.\n");
+#ifdef CONFIG_SUSPEND_FS_SYNC
+	if (suspend_fs_sync_enabled) {
+		printk(KERN_INFO "PM: Syncing filesystems ... ");
+		sys_sync();
+		printk("done.\n");
+	}
+#endif /* CONFIG_SUSPEND_FS_SYNC */
 
 	pr_debug("PM: Preparing system for %s sleep\n", pm_states[state]);
 	error = suspend_prepare(state);
-- 
1.8.3.2

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