bcache: introduce force_wake_up_gc()

Garbage collection thread starts to work when c->sectors_to_gc is
negative value, otherwise nothing will happen even the gc thread is
woken up by wake_up_gc().

force_wake_up_gc() sets c->sectors_to_gc to -1 before calling
wake_up_gc(), then gc thread may have chance to run if no one else sets
c->sectors_to_gc to a positive value before gc_should_run().

This routine can be called where the gc thread is woken up and required
to run in force.

Signed-off-by: Coly Li <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/md/bcache/btree.h

···
 	wake_up(&c->gc_wait);
 }
 
+static inline void force_wake_up_gc(struct cache_set *c)
+{
+	/*
+	 * Garbage collection thread only works when sectors_to_gc < 0,
+	 * calling wake_up_gc() won't start gc thread if sectors_to_gc is
+	 * not a nagetive value.
+	 * Therefore sectors_to_gc is set to -1 here, before waking up
+	 * gc thread by calling wake_up_gc(). Then gc_should_run() will
+	 * give a chance to permit gc thread to run. "Give a chance" means
+	 * before going into gc_should_run(), there is still possibility
+	 * that c->sectors_to_gc being set to other positive value. So
+	 * this routine won't 100% make sure gc thread will be woken up
+	 * to run.
+	 */
+	atomic_set(&c->sectors_to_gc, -1);
+	wake_up_gc(c);
+}
+
 #define MAP_DONE	0
 #define MAP_CONTINUE	1
 

drivers/md/bcache/sysfs.c

···
 		bch_cache_accounting_clear(&c->accounting);
 	}
 
-	if (attr == &sysfs_trigger_gc) {
-		/*
-		 * Garbage collection thread only works when sectors_to_gc < 0,
-		 * when users write to sysfs entry trigger_gc, most of time
-		 * they want to forcibly triger gargage collection. Here -1 is
-		 * set to c->sectors_to_gc, to make gc_should_run() give a
-		 * chance to permit gc thread to run. "give a chance" means
-		 * before going into gc_should_run(), there is still chance
-		 * that c->sectors_to_gc being set to other positive value. So
-		 * writing sysfs entry trigger_gc won't always make sure gc
-		 * thread takes effect.
-		 */
-		atomic_set(&c->sectors_to_gc, -1);
-		wake_up_gc(c);
-	}
+	if (attr == &sysfs_trigger_gc)
+		force_wake_up_gc(c);
 
 	if (attr == &sysfs_prune_cache) {
 		struct shrink_control sc;