linux-kernel - Re: [PATCH 3/5] sched: Detect call to schedule from critical entry code

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Date:   Mon, 26 Oct 2020 15:37:28 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Phil Auld <pauld@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH 3/5] sched: Detect call to schedule from critical entry
 code

On Wed, Oct 07, 2020 at 10:34:36AM +0100, Mel Gorman wrote:
> On Mon, Oct 05, 2020 at 02:26:48PM +0200, Frederic Weisbecker wrote:
> > On Mon, Oct 05, 2020 at 01:23:53PM +0200, Peter Zijlstra wrote:
> > > On Mon, Oct 05, 2020 at 12:49:17PM +0200, Frederic Weisbecker wrote:
> > > > Detect calls to schedule() between user_enter() and user_exit(). Those
> > > > are symptoms of early entry code that either forgot to protect a call
> > > > to schedule() inside exception_enter()/exception_exit() or, in the case
> > > > of HAVE_CONTEXT_TRACKING_OFFSTACK, enabled interrupts or preemption in
> > > > a wrong spot.
> > > > 
> > > > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > > > Cc: Marcelo Tosatti <mtosatti@...hat.com>
> > > > Cc: Paul E. McKenney <paulmck@...nel.org>
> > > > Cc: Peter Zijlstra <peterz@...radead.org>
> > > > Cc: Phil Auld <pauld@...hat.com>
> > > > Cc: Thomas Gleixner <tglx@...utronix.de>
> > > > ---
> > > >  kernel/sched/core.c | 1 +
> > > >  1 file changed, 1 insertion(+)
> > > > 
> > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > > index 2d95dc3f4644..d31a79e073e3 100644
> > > > --- a/kernel/sched/core.c
> > > > +++ b/kernel/sched/core.c
> > > > @@ -4295,6 +4295,7 @@ static inline void schedule_debug(struct task_struct *prev, bool preempt)
> > > >  		preempt_count_set(PREEMPT_DISABLED);
> > > >  	}
> > > >  	rcu_sleep_check();
> > > > +	WARN_ON_ONCE(ct_state() == CONTEXT_USER);
> > > 
> > > 	SCHED_WARN_ON() ?
> > 
> > Bah! That's exactly what I was looking for.
> > 
> > > No point in unconditionally polluting that path. Although, per MeL, we
> > > should probably invest in CONFIG_SCHED_DEBUG_I_MEANS_IT :/
> > 
> > Because CONFIG_SCHED_DEBUG is often used by default on distros?
> > 
> 
> SCHED_DEBUG is generally useful (e.g. figuring out weird topology problems
> on new hardware). The overhead isn't too bad when schedstats are
> disabled so it would be nice to avoid adding too much overhead via
> SCHED_DEBUG.
> 
> Other debugging options -- not so much. A lot of them are useful for
> development but there are people who request them be enabled anyway
> thinking that they improve security somehow when in reality they might,
> at best, detect a hardware issue that happens to hit a specific structure.


So we are good with SCHED_WARN_ON(), right?

I'll reissue with that.

Thanks.