linux-kernel - [PATCH] task_work: remove fifo ordering guarantee

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Message-ID: <1440816150.8932.123.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Fri, 28 Aug 2015 19:42:30 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Maciej Żenczykowski <maze@...gle.com>
Subject: [PATCH] task_work: remove fifo ordering guarantee

From: Eric Dumazet <edumazet@...gle.com>

In commit f341861fb0b ("task_work: add a scheduling point in
task_work_run()") I fixed a latency problem adding a cond_resched()
call.

Later, commit ac3d0da8f329 added yet another loop to reverse a list,
bringing back the latency spike :

I've seen in some cases this loop taking 275 ms, if for example a
process with 2,000,000 files is killed.

We could add yet another cond_resched() in the reverse loop, or we
can simply remove the reversal, as I do not think anything
would depend on order of task_work_add() submitted works.

Fixes: ac3d0da8f329 ("task_work: Make task_work_add() lockless")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: Maciej Żenczykowski <maze@...gle.com>
---
 kernel/task_work.c |   12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/kernel/task_work.c b/kernel/task_work.c
index 8727032e3a6f..53fa971d000d 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -18,6 +18,8 @@ static struct callback_head work_exited; /* all we need is ->next == NULL */
  * This is like the signal handler which runs in kernel mode, but it doesn't
  * try to wake up the @task.
  *
+ * Note: there is no ordering guarantee on works queued here.
+ *
  * RETURNS:
  * 0 if succeeds or -ESRCH.
  */
@@ -108,16 +110,6 @@ void task_work_run(void)
 		raw_spin_unlock_wait(&task->pi_lock);
 		smp_mb();
 
-		/* Reverse the list to run the works in fifo order */
-		head = NULL;
-		do {
-			next = work->next;
-			work->next = head;
-			head = work;
-			work = next;
-		} while (work);
-
-		work = head;
 		do {
 			next = work->next;
 			work->func(work);


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