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Date:   Fri, 22 Apr 2022 08:34:07 -0600
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To:     LKML <linux-kernel@vger.kernel.org>
From:   Jens Axboe <axboe@kernel.dk>
Subject: [PATCH] task_work: allow TWA_SIGNAL without a rescheduling IPI
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Some use cases don't always need an IPI when sending a TWA_SIGNAL
notification. Add TWA_SIGNAL_NO_IPI, which is just like TWA_SIGNAL,
except it doesn't send an IPI to the target task. It merely sets
TIF_NOTIFY_SIGNAL and wakes up the task.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

This is a prep patch for an io_uring change where we don't need the IPI,
and skipping it can reduce rescheduling/IPI rate by tens to hundreds of
thousands per second.

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 3c8b34876744..66b689f6cfcb 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -355,14 +355,23 @@ static inline void clear_notify_signal(void)
 	smp_mb__after_atomic();
 }
 
+/*
+ * Returns 'true' if kick_process() is needed to force a transition from
+ * user -> kernel to guarantee expedient run of TWA_SIGNAL based task_work.
+ */
+static inline bool __set_notify_signal(struct task_struct *task)
+{
+	return !test_and_set_tsk_thread_flag(task, TIF_NOTIFY_SIGNAL) &&
+	       !wake_up_state(task, TASK_INTERRUPTIBLE);
+}
+
 /*
  * Called to break out of interruptible wait loops, and enter the
  * exit_to_user_mode_loop().
  */
 static inline void set_notify_signal(struct task_struct *task)
 {
-	if (!test_and_set_tsk_thread_flag(task, TIF_NOTIFY_SIGNAL) &&
-	    !wake_up_state(task, TASK_INTERRUPTIBLE))
+	if (__set_notify_signal(task))
 		kick_process(task);
 }
 
diff --git a/include/linux/task_work.h b/include/linux/task_work.h
index 897494b597ba..795ef5a68429 100644
--- a/include/linux/task_work.h
+++ b/include/linux/task_work.h
@@ -17,6 +17,7 @@ enum task_work_notify_mode {
 	TWA_NONE,
 	TWA_RESUME,
 	TWA_SIGNAL,
+	TWA_SIGNAL_NO_IPI,
 };
 
 static inline bool task_work_pending(struct task_struct *task)
diff --git a/kernel/task_work.c b/kernel/task_work.c
index c59e1a49bc40..fa8fdd04aa17 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -13,11 +13,13 @@ static struct callback_head work_exited; /* all we need is ->next == NULL */
  *
  * Queue @work for task_work_run() below and notify the @task if @notify
  * is @TWA_RESUME or @TWA_SIGNAL. @TWA_SIGNAL works like signals, in that the
- * it will interrupt the targeted task and run the task_work. @TWA_RESUME
- * work is run only when the task exits the kernel and returns to user mode,
- * or before entering guest mode. Fails if the @task is exiting/exited and thus
- * it can't process this @work. Otherwise @work->func() will be called when the
- * @task goes through one of the aforementioned transitions, or exits.
+ * it will interrupt the targeted task and run the task_work. @TWA_SIGNAL_NO_IPI
+ * works like @TWA_SIGNAL, except it doesn't send a reschedule IPI to force the
+ * targeted task to reschedule and run task_work. @TWA_RESUME work is run only
+ * when the task exits the kernel and returns to user mode, or before entering
+ * guest mode. Fails if the @task is exiting/exited and thus it can't process
+ * this @work. Otherwise @work->func() will be called when the @task goes
+ * through one of the aforementioned transitions, or exits.
  *
  * If the targeted task is exiting, then an error is returned and the work item
  * is not queued. It's up to the caller to arrange for an alternative mechanism
@@ -53,6 +55,9 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
 	case TWA_SIGNAL:
 		set_notify_signal(task);
 		break;
+	case TWA_SIGNAL_NO_IPI:
+		__set_notify_signal(task);
+		break;
 	default:
 		WARN_ON_ONCE(1);
 		break;

-- 
Jens Axboe