Merge tag 'tif-task_work.arch-2020-12-14' of git://git.kernel.dk/linux-block

Pull TIF_NOTIFY_SIGNAL updates from Jens Axboe:
 "This sits on top of of the core entry/exit and x86 entry branch from
  the tip tree, which contains the generic and x86 parts of this work.

  Here we convert the rest of the archs to support TIF_NOTIFY_SIGNAL.

  With that done, we can get rid of JOBCTL_TASK_WORK from task_work and
  signal.c, and also remove a deadlock work-around in io_uring around
  knowing that signal based task_work waking is invoked with the sighand
  wait queue head lock.

  The motivation for this work is to decouple signal notify based
  task_work, of which io_uring is a heavy user of, from sighand. The
  sighand lock becomes a huge contention point, particularly for
  threaded workloads where it's shared between threads. Even outside of
  threaded applications it's slower than it needs to be.

  Roman Gershman <romger@amazon.com> reported that his networked
  workload dropped from 1.6M QPS at 80% CPU to 1.0M QPS at 100% CPU
  after io_uring was changed to use TIF_NOTIFY_SIGNAL. The time was all
  spent hammering on the sighand lock, showing 57% of the CPU time there
  [1].

  There are further cleanups possible on top of this. One example is
  TIF_PATCH_PENDING, where a patch already exists to use
  TIF_NOTIFY_SIGNAL instead. Hopefully this will also lead to more
  consolidation, but the work stands on its own as well"

[1] https://github.com/axboe/liburing/issues/215

* tag 'tif-task_work.arch-2020-12-14' of git://git.kernel.dk/linux-block: (28 commits)
  io_uring: remove 'twa_signal_ok' deadlock work-around
  kernel: remove checking for TIF_NOTIFY_SIGNAL
  signal: kill JOBCTL_TASK_WORK
  io_uring: JOBCTL_TASK_WORK is no longer used by task_work
  task_work: remove legacy TWA_SIGNAL path
  sparc: add support for TIF_NOTIFY_SIGNAL
  riscv: add support for TIF_NOTIFY_SIGNAL
  nds32: add support for TIF_NOTIFY_SIGNAL
  ia64: add support for TIF_NOTIFY_SIGNAL
  h8300: add support for TIF_NOTIFY_SIGNAL
  c6x: add support for TIF_NOTIFY_SIGNAL
  alpha: add support for TIF_NOTIFY_SIGNAL
  xtensa: add support for TIF_NOTIFY_SIGNAL
  arm: add support for TIF_NOTIFY_SIGNAL
  microblaze: add support for TIF_NOTIFY_SIGNAL
  hexagon: add support for TIF_NOTIFY_SIGNAL
  csky: add support for TIF_NOTIFY_SIGNAL
  openrisc: add support for TIF_NOTIFY_SIGNAL
  sh: add support for TIF_NOTIFY_SIGNAL
  um: add support for TIF_NOTIFY_SIGNAL
  ...

2 files changed, 1 insertions, 51 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index c37170655171..5736c55aaa1a 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2555,14 +2555,12 @@ bool get_signal(struct ksignal *ksig)
 	 * that the arch handlers don't all have to do it. If we get here
 	 * without TIF_SIGPENDING, just exit after running signal work.
 	 */
-#ifdef TIF_NOTIFY_SIGNAL
 	if (!IS_ENABLED(CONFIG_GENERIC_ENTRY)) {
 		if (test_thread_flag(TIF_NOTIFY_SIGNAL))
 			tracehook_notify_signal();
 		if (!task_sigpending(current))
 			return false;
 	}
-#endif
 
 	if (unlikely(uprobe_deny_signal()))
 		return false;
@@ -2576,26 +2574,6 @@ bool get_signal(struct ksignal *ksig)
 
 relock:
 	spin_lock_irq(&sighand->siglock);
-	/*
-	 * Make sure we can safely read ->jobctl() in task_work add. As Oleg
-	 * states:
-	 *
-	 * It pairs with mb (implied by cmpxchg) before READ_ONCE. So we
-	 * roughly have
-	 *
-	 *	task_work_add:				get_signal:
-	 *	STORE(task->task_works, new_work);	STORE(task->jobctl);
-	 *	mb();					mb();
-	 *	LOAD(task->jobctl);			LOAD(task->task_works);
-	 *
-	 * and we can rely on STORE-MB-LOAD [ in task_work_add].
-	 */
-	smp_store_mb(current->jobctl, current->jobctl & ~JOBCTL_TASK_WORK);
-	if (unlikely(current->task_works)) {
-		spin_unlock_irq(&sighand->siglock);
-		task_work_run();
-		goto relock;
-	}
 
 	/*
 	 * Every stopped thread goes here after wakeup. Check to see if
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 15b087286bea..9cde961875c0 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -5,34 +5,6 @@
 
 static struct callback_head work_exited; /* all we need is ->next == NULL */
 
-/*
- * TWA_SIGNAL signaling - use TIF_NOTIFY_SIGNAL, if available, as it's faster
- * than TIF_SIGPENDING as there's no dependency on ->sighand. The latter is
- * shared for threads, and can cause contention on sighand->lock. Even for
- * the non-threaded case TIF_NOTIFY_SIGNAL is more efficient, as no locking
- * or IRQ disabling is involved for notification (or running) purposes.
- */
-static void task_work_notify_signal(struct task_struct *task)
-{
-#if defined(TIF_NOTIFY_SIGNAL)
-	set_notify_signal(task);
-#else
-	unsigned long flags;
-
-	/*
-	 * Only grab the sighand lock if we don't already have some
-	 * task_work pending. This pairs with the smp_store_mb()
-	 * in get_signal(), see comment there.
-	 */
-	if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&
-	    lock_task_sighand(task, &flags)) {
-		task->jobctl |= JOBCTL_TASK_WORK;
-		signal_wake_up(task, 0);
-		unlock_task_sighand(task, &flags);
-	}
-#endif
-}
-
 /**
  * task_work_add - ask the @task to execute @work->func()
  * @task: the task which should run the callback
@@ -76,7 +48,7 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
 		set_notify_resume(task);
 		break;
 	case TWA_SIGNAL:
-		task_work_notify_signal(task);
+		set_notify_signal(task);
 		break;
 	default:
 		WARN_ON_ONCE(1);