linux-kernel - Re: ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop)

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Date:   Mon, 24 Oct 2016 05:26:39 -0800
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Waiman Long <waiman.long@....com>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Imre Deak <imre.deak@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Terry Rudd <terry.rudd@....com>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Jason Low <jason.low2@...com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Daniel Vetter <daniel.vetter@...ll.ch>, axboe@...com,
        linux-bcache@...r.kernel.org
Subject: Re: ciao set_task_state() (was Re: [PATCH -v4 6/8] locking/mutex:
 Restructure wait loop)

On Sun, Oct 23, 2016 at 06:57:26PM -0700, Davidlohr Bueso wrote:
> On Wed, 19 Oct 2016, Peter Zijlstra wrote:
> 
> > Subject: sched: Better explain sleep/wakeup
> > From: Peter Zijlstra <peterz@...radead.org>
> > Date: Wed Oct 19 15:45:27 CEST 2016
> > 
> > There were a few questions wrt how sleep-wakeup works. Try and explain
> > it more.
> > 
> > Requested-by: Will Deacon <will.deacon@....com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> > include/linux/sched.h |   52 ++++++++++++++++++++++++++++++++++----------------
> > kernel/sched/core.c   |   15 +++++++-------
> > 2 files changed, 44 insertions(+), 23 deletions(-)
> > 
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -262,20 +262,9 @@ extern char ___assert_task_state[1 - 2*!
> > #define set_task_state(tsk, state_value)			\
> > 	do {							\
> > 		(tsk)->task_state_change = _THIS_IP_;		\
> > -		smp_store_mb((tsk)->state, (state_value));		\
> > +		smp_store_mb((tsk)->state, (state_value));	\
> > 	} while (0)
> > 
> > -/*
> > - * set_current_state() includes a barrier so that the write of current->state
> > - * is correctly serialised wrt the caller's subsequent test of whether to
> > - * actually sleep:
> > - *
> > - *	set_current_state(TASK_UNINTERRUPTIBLE);
> > - *	if (do_i_need_to_sleep())
> > - *		schedule();
> > - *
> > - * If the caller does not need such serialisation then use __set_current_state()
> > - */
> > #define __set_current_state(state_value)			\
> > 	do {							\
> > 		current->task_state_change = _THIS_IP_;		\
> > @@ -284,11 +273,19 @@ extern char ___assert_task_state[1 - 2*!
> > #define set_current_state(state_value)				\
> > 	do {							\
> > 		current->task_state_change = _THIS_IP_;		\
> > -		smp_store_mb(current->state, (state_value));		\
> > +		smp_store_mb(current->state, (state_value));	\
> > 	} while (0)
> > 
> > #else
> > 
> > +/*
> > + * @tsk had better be current, or you get to keep the pieces.
> 
> That reminds me we were getting rid of the set_task_state() calls. Bcache was
> pending, being only user in the kernel that doesn't actually use current; but
> instead breaks newly (yet blocked/uninterruptible) created garbage collection
> kthread. I cannot figure out why this is done (ie purposely accounting the
> load avg. Furthermore gc kicks in in very specific scenarios obviously, such
> as as by the allocator task, so I don't see why bcache gc should want to be
> interruptible.
> 
> Kent, Jens, can we get rid of this?
> 
> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
> index 76f7534d1dd1..6e3c358b5759 100644
> --- a/drivers/md/bcache/btree.c
> +++ b/drivers/md/bcache/btree.c
> @@ -1798,7 +1798,6 @@ int bch_gc_thread_start(struct cache_set *c)
>        if (IS_ERR(c->gc_thread))
>                return PTR_ERR(c->gc_thread);
> 
> -       set_task_state(c->gc_thread, TASK_INTERRUPTIBLE);
>        return 0;
> }

Actually, that code looks broken, or at least stupid. Let me do a proper fix...