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Date: Mon, 22 Apr 2024 12:59:56 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Christoph Hellwig <hch@infradead.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>,
	Mike Snitzer <msnitzer@redhat.com>, Jens Axboe <axboe@kernel.dk>,
	Damien Le Moal <dlemoal@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Will Deacon <will@kernel.org>, Waiman Long <longman@redhat.com>,
	Guangwu Zhang <guazhang@redhat.com>, dm-devel@lists.linux.dev,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] completion: move blk_wait_io to
 kernel/sched/completion.c
Message-ID: <20240422105956.GN30852@noisy.programming.kicks-ass.net>
References: <31b118f3-bc8d-b18b-c4b9-e57d74a73f@redhat.com>
 <20240417175538.GP40213@noisy.programming.kicks-ass.net>
 <546473fd-ca4b-3c64-349d-cc739088b748@redhat.com>
 <ZiCoIHFLAzCva2lU@infradead.org>
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On Wed, Apr 17, 2024 at 09:57:04PM -0700, Christoph Hellwig wrote:
> On Wed, Apr 17, 2024 at 08:00:22PM +0200, Mikulas Patocka wrote:
> > > > +EXPORT_SYMBOL(wait_for_completion_long_io);
> > > 
> > > Urgh, why is it a sane thing to circumvent the hang check timer? 
> > 
> > The block layer already does it - the bios can have arbitrary size, so 
> > waiting for them takes arbitrary time.
> 
> And as mentioned the last few times around, I think we want a task
> state to say that task can sleep long or even forever and not propagate
> this hack even further.

A bit like TASK_NOLOAD (which is used to make TASK_IDLE work), but
different I suppose.

TASK_NOHUNG would be trivial to add ofc. But is it worth it?

Anyway, as per the other email, anything like this needs to come with a
big fat warning. You get to keep the pieces etc..

---
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3c2abbc587b4..83b25327c233 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -112,7 +112,8 @@ struct user_event_mm;
 #define TASK_FREEZABLE			0x00002000
 #define __TASK_FREEZABLE_UNSAFE	       (0x00004000 * IS_ENABLED(CONFIG_LOCKDEP))
 #define TASK_FROZEN			0x00008000
-#define TASK_STATE_MAX			0x00010000
+#define TASK_NOHUNG			0x00010000
+#define TASK_STATE_MAX			0x00020000
 
 #define TASK_ANY			(TASK_STATE_MAX-1)
 
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index b2fc2727d654..126fac835e5e 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -210,7 +210,8 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 		state = READ_ONCE(t->__state);
 		if ((state & TASK_UNINTERRUPTIBLE) &&
 		    !(state & TASK_WAKEKILL) &&
-		    !(state & TASK_NOLOAD))
+		    !(state & TASK_NOLOAD) &&
+		    !(state & TASK_NOHUNG))
 			check_hung_task(t, timeout);
 	}
  unlock: