Re: [v5.12-rc2 regression] io_uring: high CPU use after suspend-to-ram

[Jens Axboe <axboe@kernel.dk>]

On 3/9/21 6:55 PM, Kevin Locke wrote:
> With kernel 5.12-rc2 (and torvalds/master 144c79ef3353), if mpd is
> playing or paused when my system is suspended-to-ram, when the system is
> resumed mpd will consume ~200% CPU until killed.  It continues to
> produce audio and respond to pause/play commands, which do not affect
> CPU usage.  This occurs with either pulse (to PulseAudio or
> PipeWire-as-PulseAudio) or alsa audio_output.
> 
> The issue appears to have been introduced by a combination of two
> commits: 3bfe6106693b caused freeze on suspend-to-ram when mpd is paused
> or playing.  e4b4a13f4941 fixed suspend-to-ram, but introduced the high
> CPU on resume.
> 
> I attempted to further diagnose using `perf record -p $(pidof mpd)`.
> Running for about a minute after resume shows ~280 MMAP2 events and
> almost nothing else.  I'm not sure what to make of that or how to
> further investigate.
> 
> Let me know if there's anything else I can do to help diagnose/test.

The below makes it work as expected for me - but I don't quite
understand why we're continually running after the freeze. Adding Rafael
to help understand this.

Rafael, what appears to happen here from a quick look is that the io
threads are frozen fine and the system suspends. But when we resume,
signal_pending() is perpetually true, and that is why we then see the
io_wq_manager() thread just looping like crazy. Is there anything
special I need to do? Note that these are not kthreads, PF_KTHREAD is
not true. I'm guessing it may have something to do with that, but
haven't dug deeper yet.


diff --git a/fs/io-wq.c b/fs/io-wq.c
index 3d7060ba547a..0ae9ecadf295 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -591,7 +591,7 @@ static bool create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index)
 	tsk->pf_io_worker = worker;
 	worker->task = tsk;
 	set_cpus_allowed_ptr(tsk, cpumask_of_node(wqe->node));
-	tsk->flags |= PF_NOFREEZE | PF_NO_SETAFFINITY;
+	tsk->flags |= PF_NO_SETAFFINITY;
 
 	raw_spin_lock_irq(&wqe->lock);
 	hlist_nulls_add_head_rcu(&worker->nulls_node, &wqe->free_list);
@@ -709,7 +709,6 @@ static int io_wq_manager(void *data)
 		set_current_state(TASK_INTERRUPTIBLE);
 		io_wq_check_workers(wq);
 		schedule_timeout(HZ);
-		try_to_freeze();
 		if (fatal_signal_pending(current))
 			set_bit(IO_WQ_BIT_EXIT, &wq->state);
 	} while (!test_bit(IO_WQ_BIT_EXIT, &wq->state));
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 280133f3abc4..8f4128eb4aa2 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6735,7 +6735,6 @@ static int io_sq_thread(void *data)
 
 			up_read(&sqd->rw_lock);
 			schedule();
-			try_to_freeze();
 			down_read(&sqd->rw_lock);
 			list_for_each_entry(ctx, &sqd->ctx_list, sqd_list)
 				io_ring_clear_wakeup_flag(ctx);
diff --git a/kernel/fork.c b/kernel/fork.c
index d3171e8e88e5..72e444cd0ffe 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2436,6 +2436,7 @@ struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node)
 	if (!IS_ERR(tsk)) {
 		sigfillset(&tsk->blocked);
 		sigdelsetmask(&tsk->blocked, sigmask(SIGKILL));
+		tsk->flags |= PF_NOFREEZE;
 	}
 	return tsk;
 }

-- 
Jens Axboe