Re: [Regression/XFS/PM] Freeze tasks failed in xfsaild

[Yu Chen]

On Tue, Nov 14, 2017 at 03:02:32PM +1100, Dave Chinner wrote:
> On Tue, Nov 14, 2017 at 11:39:59AM +0800, Yu Chen wrote:
> > Hi Dave,
> > On Tue, Nov 14, 2017 at 09:52:16AM +1100, Dave Chinner wrote:
> > > On Mon, Nov 13, 2017 at 06:31:39PM +0800, Yu Chen wrote:
> > > > Hi all,
> > > > Currently we are running hibernation stress test on a server
> > > > and unfortunately after 48 rounds of cycling, it fails at a
> > > > early stage that, the xfs task refuses to be frozen by the system:
> > > > 
> > > > [ 1934.221653] PM: Syncing filesystems ...
> > > > [ 1934.661517] PM: done.
> > > > [ 1934.664067] Freezing user space processes ... (elapsed 0.003 seconds) done.
> > > > [ 1934.675251] OOM killer disabled.
> > > > [ 1934.724317] PM: Preallocating image memory... done (allocated 6906555 pages)
> > > > [ 1954.666378] PM: Allocated 27626220 kbytes in 19.93 seconds (1386.16 MB/s)
> > > > [ 1954.673939] Freezing remaining freezable tasks ...
> > > > [ 1974.681089] Freezing of tasks failed after 20.001 seconds (1 tasks refusing to freeze, wq_busy=0):
> > > > [ 1974.691169] xfsaild/dm-1    D    0  1362      2 0x00000080
> > > > [ 1974.697283] Call Trace:
> > > > [ 1974.700014]  __schedule+0x3be/0x830
> > > > [ 1974.703898]  schedule+0x36/0x80
> > > > [ 1974.707440]  _xfs_log_force+0x143/0x280 [xfs]
> > > > [ 1974.712295]  ? schedule_timeout+0x16b/0x350
> > > > [ 1974.716953]  ? wake_up_q+0x80/0x80
> > > > [ 1974.720752]  ? xfsaild+0x16f/0x770 [xfs]
> > > > [ 1974.725134]  xfs_log_force+0x2c/0x80 [xfs]
> > > > [ 1974.729707]  xfsaild+0x16f/0x770 [xfs]
> > > > [ 1974.733885]  kthread+0x109/0x140
> > > > [ 1974.737480]  ? kthread+0x109/0x140
> > > > [ 1974.741271]  ? xfs_trans_ail_cursor_first+0x90/0x90 [xfs]
> > > > [ 1974.747284]  ? kthread_park+0x60/0x60
> > > > [ 1974.751354]  ret_from_fork+0x25/0x30
> > > > [ 1974.755366] Restarting kernel threads ... done.
> > > > [ 1978.259907] OOM killer enabled.
> > > > [ 1978.263405] Restarting tasks ... done.
> > > > 
> > > > The reason for this failure might be that,
> > > > while the kernel thread xfsaild/dm-1 is waiting for
> > > > xfs-buf/dm-1 to wake it up, however the latter
> > > > has already been frozen, thus xfsaild/dm-1 never
> > > > has a chance to be woken up and get froze. (Although
> > > > the xfsaild/dm-1 remains in TASK_UNINTERRUPTIBLE, which
> > > > is quite similar to 'frozen'.)
> > > 
> > > Should be fixed by this commit in the for-next branch:
> > > 
> > > 0bd89676c4fe xfs: check kthread_should_stop() after the setting of task state
> > > 
> > > That should get merged into 4.15 with the next merge...
> > >
> > I did not quite catch why above commit would fix the issue here,
> > according to
> > https://git.kernel.org/pub/scm/fs/xfs/xfs-linux.git/commit/?h=for-next&id=0bd89676c4fed53b003025bc4a5200861ac5d8ef
> > it tries to address a race condition between umount and xfsaild on
> > checking the kthread_should_stop() in order not to make
> > xfsaild falling asleep indefinitely.
> 
> Argh, got my threads slightly crossed there.
> 
> > But in our case, the xfsaild is waiting for the xfs-buf to wake
> > it up, and is nothing related to the kthread_should_stop() checking
> > here, did I miss something?
> 
> Similar symptoms - the symptom that was fixed by the commit I
> mentioned was the xfsaild getting stuck in sleeping forever and so
> never seeing the KTHREAD_STOP bit - it was a "set bit vs wakeup"
> race caused by the fact that we didn't reset the state of the
> task correctly after wakeup.
Yes.
> 
> You said:
> 
> >> (Although the xfsaild/dm-1 remains in TASK_UNINTERRUPTIBLE, which
> >> is quite similar to 'frozen'.)
> 
> So from a quick look, it seemed like a similar race condition. I
> missed the *un* part of the task state, though.
> TASK_UNINTERRUPTIBLE implies waiting for IO completion, which is
> what _xfs_log_force() is doing.
> 
Yes.
> SO, follow the other branch of the discussion: hibernation needs to
> freeze filesystems so they can quiesce gracefully before the kernel
> starts shutting down the infrastructure the filesystem relies on...
>
I agree.
Before the filesystem freezing feature is merged into
upstream, wouldn't it be nice if we have some compromise
workaround for such kind of issues: how about treat the
always-sleeping tasks as frozen? They are safe to be
regarded as frozen because they do nothing.
Here's a draft patch to get it done, and it can be
optimized if the direction is acceptible.

diff --git a/kernel/power/process.c b/kernel/power/process.c
index 7381d49a44db..93e123a58558 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -100,8 +100,28 @@ static int try_to_freeze_tasks(bool user_only)
 			read_lock(&tasklist_lock);
 			for_each_process_thread(g, p) {
 				if (p != current && !freezer_should_skip(p)
-				    && freezing(p) && !frozen(p))
-					sched_show_task(p);
+				    && freezing(p) && !frozen(p)) {
+					unsigned long cnt_ctx;
+
+					cnt_ctx = p->nvcsw + p->nivcsw;
+					msleep(MSEC_PER_SEC);
+					/* If the task keeps asleep for 1000 ms
+					 * (actually should be:
+					 * freeze_timeout_msecs+1000ms in theory)
+					 * AKA, there is no context switch for the
+					 * task during this period, we predict this
+					 * task is not likely to do any work in the
+					 * future and we can treat it as frozen.
+					 */
+					if ((p->state & TASK_NORMAL) &&
+					   (cnt_ctx == (p->nvcsw + p->nivcsw))) {
+						pr_err("(%s %c) is sleeping and safe to be treated as frozen\n",
+							p->comm, task_state_to_char(p));
+						todo = 0;
+					} else {
+						sched_show_task(p);
+					}
+				}
 			}
 			read_unlock(&tasklist_lock);
 		}
-- 
2.13.5