Re: [peterz-queue:sched/core] [sched/fair] 420356c350: WARNING:at_kernel/sched/core.c:#__might_sleep

[Chen Yu <yu.c.chen@intel.com>]

On 2024-08-22 at 17:49:23 +0200, Peter Zijlstra wrote:
> On Mon, Aug 19, 2024 at 12:44:39PM +0800, Chen Yu wrote:
> > On 2024-08-17 at 11:33:29 +0200, Peter Zijlstra wrote:
> > > On Fri, Aug 16, 2024 at 05:15:12PM +0800, kernel test robot wrote:
> > > > kernel test robot noticed "WARNING:at_kernel/sched/core.c:#__might_sleep" on:
> > > > 
> > > > commit: 420356c3504091f0f6021974389df7c58f365dad ("sched/fair: Implement delayed dequeue")
> > > > https://git.kernel.org/cgit/linux/kernel/git/peterz/queue.git sched/core
> > > 
> > > > [   86.252370][  T674] ------------[ cut here ]------------
> > > > [ 86.252945][ T674] do not call blocking ops when !TASK_RUNNING; state=1 set at kthread_worker_fn (kernel/kthread.c:?) 
> > > > [ 86.254001][ T674] WARNING: CPU: 1 PID: 674 at kernel/sched/core.c:8469 __might_sleep (kernel/sched/core.c:8465) 
> > > 
> > > > [ 86.283398][ T674] ? handle_bug (arch/x86/kernel/traps.c:239) 
> > > > [ 86.283995][ T674] ? exc_invalid_op (arch/x86/kernel/traps.c:260) 
> > > > [ 86.284787][ T674] ? asm_exc_invalid_op (arch/x86/include/asm/idtentry.h:621) 
> > > > [ 86.285682][ T674] ? __might_sleep (kernel/sched/core.c:8465) 
> > > > [ 86.286380][ T674] ? __might_sleep (kernel/sched/core.c:8465) 
> > > > [ 86.287116][ T674] kthread_worker_fn (include/linux/kernel.h:73 include/linux/freezer.h:53 kernel/kthread.c:851) 
> > > > [ 86.287701][ T674] ? kthread_worker_fn (kernel/kthread.c:?) 
> > > > [ 86.288138][ T674] kthread (kernel/kthread.c:391) 
> > > > [ 86.288482][ T674] ? __cfi_kthread_worker_fn (kernel/kthread.c:803) 
> > > > [ 86.288951][ T674] ? __cfi_kthread (kernel/kthread.c:342) 
> > > > [ 86.289560][ T674] ret_from_fork (arch/x86/kernel/process.c:153) 
> > > > [ 86.290162][ T674] ? __cfi_kthread (kernel/kthread.c:342) 
> > > > [ 86.291465][ T674] ret_from_fork_asm (arch/x86/entry/entry_64.S:254) 
> > > 
> > > AFAICT this is a pre-existing issue. Notably that all transcribes to:
> > > 
> > > kthread_worker_fn()
> > >   ...
> > > repeat:
> > >   set_current_state(TASK_INTERRUPTIBLE);
> > >   ...
> > >   if (work) { // false
> > >     __set_current_state(TASK_RUNNING);
> > >     ...
> > >   } else if (!freezing(current)) // false -- we are freezing
> > >     schedule();
> > > 
> > >   // so state really is still TASK_INTERRUPTIBLE here
> > >   try_to_freeze()
> > >     might_sleep() <--- boom, per the above.
> > >
> > 
> > Would the following fix make sense?
> 
> Yeah, that looks fine. Could you write it up as a proper patch please?
>

Yes, it should be a race condition in theory and I've sent a patch here:
https://lore.kernel.org/lkml/20240819141551.111610-1-yu.c.chen@intel.com/
And Andrew has given some comments on it.

However, after I did some further investigation, this warning seems to
not be directly related to task freeze, but has connection with the
delay dequeue. I'm planning to add debug patch and investigate the
symptom in 0day's environment, will send the finding later.

thanks,
Chenyu

> > 
> > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > index f7be976ff88a..09850b2109c9 100644
> > --- a/kernel/kthread.c
> > +++ b/kernel/kthread.c
> > @@ -848,6 +848,12 @@ int kthread_worker_fn(void *worker_ptr)
> >  	} else if (!freezing(current))
> >  		schedule();
> >  
> > +	/*
> > +	 * Explictly set the running state in case we are being frozen
> > +	 * and skip the schedule() above. try_to_freeze() expects the
> > +	 * current task to be in running state.
> > +	 */
> > +	__set_current_state(TASK_RUNNING);
> >  	try_to_freeze();
> >  	cond_resched();
> >  	goto repeat;
> > -- 
> > 2.25.1
> > 
> > Hi Oliver,
> > Could you please help check if above change would make the warning go away?
> > 
> > thanks,
> > Chenyu