Re: [PATCH 2/4] kernel.h: Add non_block_start/end()

From: Daniel Vetter <daniel@ffwll.ch>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Michal Hocko" <mhocko@kernel.org>,
	"Daniel Vetter" <daniel.vetter@ffwll.ch>,
	"Intel Graphics Development" <intel-gfx@lists.freedesktop.org>,
	"DRI Development" <dri-devel@lists.freedesktop.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org, "Andrew Morton" <akpm@linux-foundation.org>,
	"David Rientjes" <rientjes@google.com>,
	"Christian König" <christian.koenig@amd.com>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Daniel Vetter" <daniel.vetter@intel.com>
Subject: Re: [PATCH 2/4] kernel.h: Add non_block_start/end()
Date: Wed, 12 Dec 2018 11:26:56 +0100	[thread overview]
Message-ID: <20181212102656.GS21184@phenom.ffwll.local> (raw)
In-Reply-To: <20181210163009.GR5289@hirez.programming.kicks-ass.net>

On Mon, Dec 10, 2018 at 05:30:09PM +0100, Peter Zijlstra wrote:
> On Mon, Dec 10, 2018 at 05:20:10PM +0100, Michal Hocko wrote:
> > > OK, no real objections to the thing.  Just so long we're all on the same
> > > page as to what it does and doesn't do ;-)
> > 
> > I am not really sure whether there are other potential users besides
> > this one and whether the check as such is justified.
> 
> It's a debug option...
> 
> > > I suppose you could extend the check to include schedule_debug() as
> > > well, maybe something like:
> > 
> > Do you mean to make the check cheaper?
> 
> Nah, so the patch only touched might_sleep(), the below touches
> schedule().
> 
> If there were a patch that hits schedule() without going through a
> might_sleep() (rare in practise I think, but entirely possible) then you
> won't get a splat without something like the below on top.

We have a bunch of schedule() calls in i915, for e.g. waiting for multiple
events at the same time (when we want to unblock if any of them fire). And
there's no might_sleep in these cases afaict. Adding the check in
schedule() sounds useful, I'll include your snippet in v2. Plus try a bit
better to explain in the commit message why Michal suggested these.

Thanks, Daniel

> 
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index f66920173370..b1aaa278f1af 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -3278,13 +3278,18 @@ static noinline void __schedule_bug(struct task_struct *prev)
> > >  /*
> > >   * Various schedule()-time debugging checks and statistics:
> > >   */
> > > -static inline void schedule_debug(struct task_struct *prev)
> > > +static inline void schedule_debug(struct task_struct *prev, bool preempt)
> > >  {
> > >  #ifdef CONFIG_SCHED_STACK_END_CHECK
> > >  	if (task_stack_end_corrupted(prev))
> > >  		panic("corrupted stack end detected inside scheduler\n");
> > >  #endif
> > >  
> > > +#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
> > > +	if (!preempt && prev->state && prev->non_block_count)
> > > +		// splat
> > > +#endif
> > > +
> > >  	if (unlikely(in_atomic_preempt_off())) {
> > >  		__schedule_bug(prev);
> > >  		preempt_count_set(PREEMPT_DISABLED);
> > > @@ -3391,7 +3396,7 @@ static void __sched notrace __schedule(bool preempt)
> > >  	rq = cpu_rq(cpu);
> > >  	prev = rq->curr;
> > >  
> > > -	schedule_debug(prev);
> > > +	schedule_debug(prev, preempt);
> > >  
> > >  	if (sched_feat(HRTICK))
> > >  		hrtick_clear(rq);
> > 
> > -- 
> > Michal Hocko
> > SUSE Labs

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch