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Date: Tue, 23 Apr 2024 14:36:45 +0200 (CEST)
From: Mikulas Patocka <mpatocka@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
cc: Christoph Hellwig <hch@infradead.org>, 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
In-Reply-To: <20240422105956.GN30852@noisy.programming.kicks-ass.net>
Message-ID: <8ab5c16e-154-c66-d167-5ce9eb6ea331@redhat.com>
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> <20240422105956.GN30852@noisy.programming.kicks-ass.net>
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On Mon, 22 Apr 2024, Peter Zijlstra wrote:

> 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..

This seems better than the blk_wait_io hack.

Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>

> ---
> 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:
>