linux-kernel - [PATCH AUTOSEL 3.18 54/98] nfs: fix high load average due to callback thread sleeping

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Message-Id: <20181025141853.214051-54-sashal@kernel.org>
Date:   Thu, 25 Oct 2018 10:18:09 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     stable@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Jeff Layton <jlayton@...chiereds.net>,
        Jeff Layton <jeff.layton@...marydata.com>,
        Trond Myklebust <trond.myklebust@...marydata.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 3.18 54/98] nfs: fix high load average due to callback thread sleeping

From: Jeff Layton <jlayton@...chiereds.net>

[ Upstream commit 5d05e54af3cdbb13cf19c557ff2184781b91a22c ]

Chuck pointed out a problem that crept in with commit 6ffa30d3f734 (nfs:
don't call blocking operations while !TASK_RUNNING). Linux counts tasks
in uninterruptible sleep against the load average, so this caused the
system's load average to be pinned at at least 1 when there was a
NFSv4.1+ mount active.

Not a huge problem, but it's probably worth fixing before we get too
many complaints about it. This patch converts the code back to use
TASK_INTERRUPTIBLE sleep, simply has it flush any signals on each loop
iteration. In practice no one should really be signalling this thread at
all, so I think this is reasonably safe.

With this change, there's also no need to game the hung task watchdog so
we can also convert the schedule_timeout call back to a normal schedule.

Cc: <stable@...r.kernel.org>
Reported-by: Chuck Lever <chuck.lever@...cle.com>
Signed-off-by: Jeff Layton <jeff.layton@...marydata.com>
Tested-by: Chuck Lever <chuck.lever@...cle.com>
Fixes: commit 6ffa30d3f734 (“nfs: don't call blocking . . .”)
Signed-off-by: Trond Myklebust <trond.myklebust@...marydata.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 fs/nfs/callback.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
index 3b8cdb8298c4..a78558a25035 100644
--- a/fs/nfs/callback.c
+++ b/fs/nfs/callback.c
@@ -128,7 +128,7 @@ nfs41_callback_svc(void *vrqstp)
 		if (try_to_freeze())
 			continue;
 
-		prepare_to_wait(&serv->sv_cb_waitq, &wq, TASK_UNINTERRUPTIBLE);
+		prepare_to_wait(&serv->sv_cb_waitq, &wq, TASK_INTERRUPTIBLE);
 		spin_lock_bh(&serv->sv_cb_lock);
 		if (!list_empty(&serv->sv_cb_list)) {
 			req = list_first_entry(&serv->sv_cb_list,
@@ -142,10 +142,10 @@ nfs41_callback_svc(void *vrqstp)
 				error);
 		} else {
 			spin_unlock_bh(&serv->sv_cb_lock);
-			/* schedule_timeout to game the hung task watchdog */
-			schedule_timeout(60 * HZ);
+			schedule();
 			finish_wait(&serv->sv_cb_waitq, &wq);
 		}
+		flush_signals(current);
 	}
 	return 0;
 }
-- 
2.17.1