Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc:     Oleg Nesterov <oleg@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
From:   Jens Axboe <axboe@kernel.dk>
Subject: [PATCH] task_work: only grab task signal lock when needed
Message-ID: <0028d3ea-4d05-405f-b457-75c83d381d89@kernel.dk>
Date:   Tue, 11 Aug 2020 08:25:20 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.10.0
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

If JOBCTL_TASK_WORK is already set on the targeted task, then we need
not go through {lock,unlock}_task_sighand() to set it again and queue
a signal wakeup. This is safe as we're checking it _after adding the
new task_work with cmpxchg().

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

---

Tested this with an intensive task_work based io_uring workload, and
the benefits are quite large.

diff --git a/kernel/task_work.c b/kernel/task_work.c
index 5c0848ca1287..cbf8cab6e864 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -42,7 +42,8 @@ task_work_add(struct task_struct *task, struct callback_head *work, int notify)
 		set_notify_resume(task);
 		break;
 	case TWA_SIGNAL:
-		if (lock_task_sighand(task, &flags)) {
+		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);
-- 
Jens Axboe