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>

2 files changed, 12 insertions, 0 deletions

diff --git a/kernel/power/Kconfig b/kernel/power/Kconfig
index 9e302315e33d..02e8dfaa1ce2 100644
--- a/kernel/power/Kconfig
+++ b/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
 
diff --git a/kernel/power/suspend.c b/kernel/power/suspend.c
index 53266b729fd9..7e4cda4a8dd9 100644
--- a/kernel/power/suspend.c
+++ b/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);