PM / suspend: make sync() on suspend-to-RAM build-time optional

The Linux kernel suspend path has traditionally invoked sys_sync()
before freezing user threads.

But sys_sync() can be expensive, and some user-space OS's do not want
the kernel to pay the cost of sys_sync() on every suspend -- preferring
invoke sync() from user-space if/when they want it.

So make sys_sync on suspend build-time optional.

The default is unchanged.

Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/power/Kconfig

@@ -18,6 +18,16 @@ config SUSPEND_FREEZER
 
 	  Turning OFF this setting is NOT recommended! If in doubt, say Y.
 
+config SUSPEND_SKIP_SYNC
+	bool "Skip kernel's sys_sync() on suspend to RAM/standby"
+	depends on SUSPEND
+	depends on EXPERT
+	help
+	  Skip the kernel sys_sync() before freezing user processes.
+	  Some systems prefer not to pay this cost on every invocation
+	  of suspend, or they are content with invoking sync() from
+	  user-space before invoking suspend.  Say Y if that's your case.
+
 config HIBERNATE_CALLBACKS
 	bool
 

kernel/power/suspend.c

@@ -484,11 +484,13 @@ static int enter_state(suspend_state_t state)
 	if (state == PM_SUSPEND_FREEZE)
 		freeze_begin();
 
+#ifndef CONFIG_SUSPEND_SKIP_SYNC
 	trace_suspend_resume(TPS("sync_filesystems"), 0, true);
 	printk(KERN_INFO "PM: Syncing filesystems ... ");
 	sys_sync();
 	printk("done.\n");
 	trace_suspend_resume(TPS("sync_filesystems"), 0, false);
+#endif
 
 	pr_debug("PM: Preparing system for sleep (%s)\n", pm_states[state]);
 	error = suspend_prepare(state);