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Date: Wed, 17 Apr 2024 20:00:22 +0200 (CEST)
From: Mikulas Patocka <mpatocka@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
cc: 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: <20240417175538.GP40213@noisy.programming.kicks-ass.net>
Message-ID: <546473fd-ca4b-3c64-349d-cc739088b748@redhat.com>
References: <31b118f3-bc8d-b18b-c4b9-e57d74a73f@redhat.com> <20240417175538.GP40213@noisy.programming.kicks-ass.net>
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On Wed, 17 Apr 2024, Peter Zijlstra wrote:

> On Wed, Apr 17, 2024 at 07:49:17PM +0200, Mikulas Patocka wrote:
> > Index: linux-2.6/kernel/sched/completion.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/sched/completion.c	2024-04-17 19:41:14.000000000 +0200
> > +++ linux-2.6/kernel/sched/completion.c	2024-04-17 19:41:14.000000000 +0200
> > @@ -290,6 +290,26 @@ wait_for_completion_killable_timeout(str
> >  EXPORT_SYMBOL(wait_for_completion_killable_timeout);
> >  
> >  /**
> > + * wait_for_completion_long_io - waits for completion of a task
> > + * @x:  holds the state of this particular completion
> > + *
> > + * This is like wait_for_completion_io, but it doesn't warn if the wait takes
> > + * too long.
> > + */
> > +void wait_for_completion_long_io(struct completion *x)
> > +{
> > +	/* Prevent hang_check timer from firing at us during very long I/O */
> > +	unsigned long timeout = sysctl_hung_task_timeout_secs * HZ / 2;
> > +
> > +	if (timeout)
> > +		while (!wait_for_completion_io_timeout(x, timeout))
> > +			;
> > +	else
> > +		wait_for_completion_io(x);
> > +}
> > +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.

Mikulas