sched/core: Remove set_task_state()

This is a nasty interface and setting the state of a foreign task must
not be done. As of the following commit:

  be628be0956 ("bcache: Make gc wakeup sane, remove set_task_state()")

... everyone in the kernel calls set_task_state() with current, allowing
the helper to be removed.

However, as the comment indicates, it is still around for those archs
where computing current is more expensive than using a pointer, at least
in theory. An important arch that is affected is arm64, however this has
been addressed now [1] and performance is up to par making no difference
with either calls.

Of all the callers, if any, it's the locking bits that would care most
about this -- ie: we end up passing a tsk pointer to a lot of the lock
slowpath, and setting ->state on that. The following numbers are based
on two tests: a custom ad-hoc microbenchmark that just measures
latencies (for ~65 million calls) between get_task_state() vs
get_current_state().

Secondly for a higher overview, an unlink microbenchmark was used,
which pounds on a single file with open, close,unlink combos with
increasing thread counts (up to 4x ncpus). While the workload is quite
unrealistic, it does contend a lot on the inode mutex or now rwsem.

[1] https://lkml.kernel.org/r/1483468021-8237-1-git-send-email-mark.rutland@arm.com

== 1. x86-64 ==

Avg runtime set_task_state():    601 msecs
Avg runtime set_current_state(): 552 msecs

                                            vanilla                 dirty
Hmean    unlink1-processes-2      36089.26 (  0.00%)    38977.33 (  8.00%)
Hmean    unlink1-processes-5      28555.01 (  0.00%)    29832.55 (  4.28%)
Hmean    unlink1-processes-8      37323.75 (  0.00%)    44974.57 ( 20.50%)
Hmean    unlink1-processes-12     43571.88 (  0.00%)    44283.01 (  1.63%)
Hmean    unlink1-processes-21     34431.52 (  0.00%)    38284.45 ( 11.19%)
Hmean    unlink1-processes-30     34813.26 (  0.00%)    37975.17 (  9.08%)
Hmean    unlink1-processes-48     37048.90 (  0.00%)    39862.78 (  7.59%)
Hmean    unlink1-processes-79     35630.01 (  0.00%)    36855.30 (  3.44%)
Hmean    unlink1-processes-110    36115.85 (  0.00%)    39843.91 ( 10.32%)
Hmean    unlink1-processes-141    32546.96 (  0.00%)    35418.52 (  8.82%)
Hmean    unlink1-processes-172    34674.79 (  0.00%)    36899.21 (  6.42%)
Hmean    unlink1-processes-203    37303.11 (  0.00%)    36393.04 ( -2.44%)
Hmean    unlink1-processes-224    35712.13 (  0.00%)    36685.96 (  2.73%)

== 2. ppc64le ==

Avg runtime set_task_state():  938 msecs
Avg runtime set_current_state: 940 msecs

                                            vanilla                 dirty
Hmean    unlink1-processes-2      19269.19 (  0.00%)    30704.50 ( 59.35%)
Hmean    unlink1-processes-5      20106.15 (  0.00%)    21804.15 (  8.45%)
Hmean    unlink1-processes-8      17496.97 (  0.00%)    17243.28 ( -1.45%)
Hmean    unlink1-processes-12     14224.15 (  0.00%)    17240.21 ( 21.20%)
Hmean    unlink1-processes-21     14155.66 (  0.00%)    15681.23 ( 10.78%)
Hmean    unlink1-processes-30     14450.70 (  0.00%)    15995.83 ( 10.69%)
Hmean    unlink1-processes-48     16945.57 (  0.00%)    16370.42 ( -3.39%)
Hmean    unlink1-processes-79     15788.39 (  0.00%)    14639.27 ( -7.28%)
Hmean    unlink1-processes-110    14268.48 (  0.00%)    14377.40 (  0.76%)
Hmean    unlink1-processes-141    14023.65 (  0.00%)    16271.69 ( 16.03%)
Hmean    unlink1-processes-172    13417.62 (  0.00%)    16067.55 ( 19.75%)
Hmean    unlink1-processes-203    15293.08 (  0.00%)    15440.40 (  0.96%)
Hmean    unlink1-processes-234    13719.32 (  0.00%)    16190.74 ( 18.01%)
Hmean    unlink1-processes-265    16400.97 (  0.00%)    16115.22 ( -1.74%)
Hmean    unlink1-processes-296    14388.60 (  0.00%)    16216.13 ( 12.70%)
Hmean    unlink1-processes-320    15771.85 (  0.00%)    15905.96 (  0.85%)

x86-64 (known to be fast for get_current()/this_cpu_read_stable() caching)
and ppc64 (with paca) show similar improvements in the unlink microbenches.
The small delta for ppc64 (2ms), does not represent the gains on the unlink
runs. In the case of x86, there was a decent amount of variation in the
latency runs, but always within a 20 to 50ms increase), ppc was more constant.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dave@stgolabs.net
Cc: mark.rutland@arm.com
Link: http://lkml.kernel.org/r/1483479794-14013-5-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/um/drivers/random.c

@@ -76,7 +76,7 @@ static ssize_t rng_dev_read (struct file *filp, char __user *buf, size_t size,
 			add_sigio_fd(random_fd);
 
 			add_wait_queue(&host_read_wait, &wait);
-			set_task_state(current, TASK_INTERRUPTIBLE);
+			set_current_state(TASK_INTERRUPTIBLE);
 
 			schedule();
 			remove_wait_queue(&host_read_wait, &wait);

drivers/md/dm-bufio.c

@@ -794,7 +794,7 @@ static void __wait_for_free_buffer(struct dm_bufio_client *c)
 	DECLARE_WAITQUEUE(wait, current);
 
 	add_wait_queue(&c->free_buffer_wait, &wait);
-	set_task_state(current, TASK_UNINTERRUPTIBLE);
+	set_current_state(TASK_UNINTERRUPTIBLE);
 	dm_bufio_unlock(c);
 
 	io_schedule();

drivers/md/dm-crypt.c

@@ -1210,14 +1210,14 @@ continue_locked:
 		spin_unlock_irq(&cc->write_thread_wait.lock);
 
 		if (unlikely(kthread_should_stop())) {
-			set_task_state(current, TASK_RUNNING);
+			set_current_state(TASK_RUNNING);
 			remove_wait_queue(&cc->write_thread_wait, &wait);
 			break;
 		}
 
 		schedule();
 
-		set_task_state(current, TASK_RUNNING);
+		set_current_state(TASK_RUNNING);
 		spin_lock_irq(&cc->write_thread_wait.lock);
 		__remove_wait_queue(&cc->write_thread_wait, &wait);
 		goto continue_locked;

drivers/md/persistent-data/dm-block-manager.c

@@ -120,7 +120,7 @@ static int __check_holder(struct block_lock *lock)
 static void __wait(struct waiter *w)
 {
 	for (;;) {
-		set_task_state(current, TASK_UNINTERRUPTIBLE);
+		set_current_state(TASK_UNINTERRUPTIBLE);
 
 		if (!w->task)
 			break;
@@ -128,7 +128,7 @@ static void __wait(struct waiter *w)
 		schedule();
 	}
 
-	set_task_state(current, TASK_RUNNING);
+	set_current_state(TASK_RUNNING);
 }
 
 static void __wake_waiter(struct waiter *w)

drivers/staging/lustre/lnet/libcfs/linux/linux-debug.c

@@ -107,7 +107,7 @@ void __noreturn lbug_with_loc(struct libcfs_debug_msg_data *msgdata)
 		libcfs_debug_dumplog();
 	if (libcfs_panic_on_lbug)
 		panic("LBUG");
-	set_task_state(current, TASK_UNINTERRUPTIBLE);
+	set_current_state(TASK_UNINTERRUPTIBLE);
 	while (1)
 		schedule();
 }

drivers/tty/tty_ldsem.c

@@ -231,7 +231,7 @@ down_read_failed(struct ld_semaphore *sem, long count, long timeout)
 
 	/* wait to be given the lock */
 	for (;;) {
-		set_task_state(current, TASK_UNINTERRUPTIBLE);
+		set_current_state(TASK_UNINTERRUPTIBLE);
 
 		if (!waiter.task)
 			break;
@@ -240,7 +240,7 @@ down_read_failed(struct ld_semaphore *sem, long count, long timeout)
 		timeout = schedule_timeout(timeout);
 	}
 
-	__set_task_state(current, TASK_RUNNING);
+	__set_current_state(TASK_RUNNING);
 
 	if (!timeout) {
 		/* lock timed out but check if this task was just
@@ -289,14 +289,14 @@ down_write_failed(struct ld_semaphore *sem, long count, long timeout)
 
 	waiter.task = current;
 
-	set_task_state(current, TASK_UNINTERRUPTIBLE);
+	set_current_state(TASK_UNINTERRUPTIBLE);
 	for (;;) {
 		if (!timeout)
 			break;
 		raw_spin_unlock_irq(&sem->wait_lock);
 		timeout = schedule_timeout(timeout);
 		raw_spin_lock_irq(&sem->wait_lock);
-		set_task_state(current, TASK_UNINTERRUPTIBLE);
+		set_current_state(TASK_UNINTERRUPTIBLE);
 		locked = writer_trylock(sem);
 		if (locked)
 			break;
@@ -307,7 +307,7 @@ down_write_failed(struct ld_semaphore *sem, long count, long timeout)
 	list_del(&waiter.list);
 	raw_spin_unlock_irq(&sem->wait_lock);
 
-	__set_task_state(current, TASK_RUNNING);
+	__set_current_state(TASK_RUNNING);
 
 	/* lock wait may have timed out */
 	if (!locked)

include/linux/sched.h

@@ -227,7 +227,7 @@ extern void proc_sched_set_task(struct task_struct *p);
 extern char ___assert_task_state[1 - 2*!!(
 		sizeof(TASK_STATE_TO_CHAR_STR)-1 != ilog2(TASK_STATE_MAX)+1)];
 
-/* Convenience macros for the sake of set_task_state */
+/* Convenience macros for the sake of set_current_state */
 #define TASK_KILLABLE		(TASK_WAKEKILL | TASK_UNINTERRUPTIBLE)
 #define TASK_STOPPED		(TASK_WAKEKILL | __TASK_STOPPED)
 #define TASK_TRACED		(TASK_WAKEKILL | __TASK_TRACED)
@@ -254,17 +254,6 @@ extern char ___assert_task_state[1 - 2*!!(
 
 #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
 
-#define __set_task_state(tsk, state_value)			\
-	do {							\
-		(tsk)->task_state_change = _THIS_IP_;		\
-		(tsk)->state = (state_value);			\
-	} while (0)
-#define set_task_state(tsk, state_value)			\
-	do {							\
-		(tsk)->task_state_change = _THIS_IP_;		\
-		smp_store_mb((tsk)->state, (state_value));	\
-	} while (0)
-
 #define __set_current_state(state_value)			\
 	do {							\
 		current->task_state_change = _THIS_IP_;		\
@@ -277,20 +266,6 @@ extern char ___assert_task_state[1 - 2*!!(
 	} while (0)
 
 #else
-
-/*
- * @tsk had better be current, or you get to keep the pieces.
- *
- * The only reason is that computing current can be more expensive than
- * using a pointer that's already available.
- *
- * Therefore, see set_current_state().
- */
-#define __set_task_state(tsk, state_value)		\
-	do { (tsk)->state = (state_value); } while (0)
-#define set_task_state(tsk, state_value)		\
-	smp_store_mb((tsk)->state, (state_value))
-
 /*
  * set_current_state() includes a barrier so that the write of current->state
  * is correctly serialised wrt the caller's subsequent test of whether to

kernel/exit.c

@@ -501,12 +501,12 @@ static void exit_mm(void)
 			complete(&core_state->startup);
 
 		for (;;) {
-			set_task_state(current, TASK_UNINTERRUPTIBLE);
+			set_current_state(TASK_UNINTERRUPTIBLE);
 			if (!self.task) /* see coredump_finish() */
 				break;
 			freezable_schedule();
 		}
-		__set_task_state(current, TASK_RUNNING);
+		__set_current_state(TASK_RUNNING);
 		down_read(&mm->mmap_sem);
 	}
 	atomic_inc(&mm->mm_count);

kernel/locking/mutex.c

@@ -666,7 +666,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 
 	lock_contended(&lock->dep_map, ip);
 
-	set_task_state(current, state);
+	set_current_state(state);
 	for (;;) {
 		/*
 		 * Once we hold wait_lock, we're serialized against
@@ -701,7 +701,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
 		}
 
-		set_task_state(current, state);
+		set_current_state(state);
 		/*
 		 * Here we order against unlock; we must either see it change
 		 * state back to RUNNING and fall through the next schedule(),
@@ -715,7 +715,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 	}
 	spin_lock_mutex(&lock->wait_lock, flags);
 acquired:
-	__set_task_state(current, TASK_RUNNING);
+	__set_current_state(TASK_RUNNING);
 
 	mutex_remove_waiter(lock, &waiter, current);
 	if (likely(list_empty(&lock->wait_list)))
@@ -735,7 +735,7 @@ skip_wait:
 	return 0;
 
 err:
-	__set_task_state(current, TASK_RUNNING);
+	__set_current_state(TASK_RUNNING);
 	mutex_remove_waiter(lock, &waiter, current);
 	spin_unlock_mutex(&lock->wait_lock, flags);
 	debug_mutex_free_waiter(&waiter);

kernel/locking/rwsem-spinlock.c

@@ -139,7 +139,7 @@ void __sched __down_read(struct rw_semaphore *sem)
 		goto out;
 	}
 
-	set_task_state(current, TASK_UNINTERRUPTIBLE);
+	set_current_state(TASK_UNINTERRUPTIBLE);
 
 	/* set up my own style of waitqueue */
 	waiter.task = current;
@@ -156,10 +156,10 @@ void __sched __down_read(struct rw_semaphore *sem)
 		if (!waiter.task)
 			break;
 		schedule();
-		set_task_state(current, TASK_UNINTERRUPTIBLE);
+		set_current_state(TASK_UNINTERRUPTIBLE);
 	}
 
-	__set_task_state(current, TASK_RUNNING);
+	__set_current_state(TASK_RUNNING);
  out:
 	;
 }
@@ -216,7 +216,7 @@ int __sched __down_write_common(struct rw_semaphore *sem, int state)
 			ret = -EINTR;
 			goto out;
 		}
-		set_task_state(current, state);
+		set_current_state(state);
 		raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 		schedule();
 		raw_spin_lock_irqsave(&sem->wait_lock, flags);

kernel/locking/rwsem-xadd.c

@@ -253,13 +253,13 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 
 	/* wait to be given the lock */
 	while (true) {
-		set_task_state(current, TASK_UNINTERRUPTIBLE);
+		set_current_state(TASK_UNINTERRUPTIBLE);
 		if (!waiter.task)
 			break;
 		schedule();
 	}
 
-	__set_task_state(current, TASK_RUNNING);
+	__set_current_state(TASK_RUNNING);
 	return sem;
 }
 EXPORT_SYMBOL(rwsem_down_read_failed);

kernel/locking/semaphore.c

@@ -215,7 +215,7 @@ static inline int __sched __down_common(struct semaphore *sem, long state,
 			goto interrupted;
 		if (unlikely(timeout <= 0))
 			goto timed_out;
-		__set_task_state(current, state);
+		__set_current_state(state);
 		raw_spin_unlock_irq(&sem->lock);
 		timeout = schedule_timeout(timeout);
 		raw_spin_lock_irq(&sem->lock);