Re: [PATCH] lib: Check for migrate_disable only on SMP systems

[Scott Wood <swood@redhat.com>]

On Fri, 2019-12-06 at 20:39 +0100, Daniel Wagner wrote:
> Hi Scott,
> 
> On Fri, Dec 06, 2019 at 12:49:12PM -0600, Scott Wood wrote:
> > On Fri, 2019-12-06 at 15:21 +0100, Daniel Wagner wrote:
> > > In case the kernel configuration is UP is, the migrate_disable member
> > > in task_struct is missing.
> > > 
> > > linux/lib/smp_processor_id.c: In function ‘check_preemption_disabled’:
> > > linux/lib/smp_processor_id.c:26:13: error: ‘struct task_struct’ has no
> > > member named ‘migrate_disable’
> > >     26 |  if (current->migrate_disable)
> > >        |             ^~
> > > 
> > > Fixes: 425c5b38779a ("sched: Lazy migrate_disable processing")
> > > Cc: Scott Wood <swood@redhat.com>
> > > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > > Signed-off-by: Daniel Wagner <dwagner@suse.de>
> > > ---
> > >  lib/smp_processor_id.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/lib/smp_processor_id.c b/lib/smp_processor_id.c
> > > index 5f2618d346c4..a914f73e3652 100644
> > > --- a/lib/smp_processor_id.c
> > > +++ b/lib/smp_processor_id.c
> > > @@ -23,8 +23,10 @@ unsigned int check_preemption_disabled(const char
> > > *what1, const char *what2)
> > >  	 * Kernel threads bound to a single CPU can safely use
> > >  	 * smp_processor_id():
> > >  	 */
> > > +#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT_BASE)
> > >  	if (current->migrate_disable)
> > >  		goto out;
> > > +#endif
> > 
> > Won't this give false positives on UP with CONFIG_PREEMPT_RT_BASE and
> > CONFIG_DEBUG_PREEMPT? If we have CONFIG_SCHED_DEBUG then we can still
> > check
> > migrate_disable.
> 
> Ohhh, I see what you mean. I didn't realize that migrate_disable is
> also available with CONFIG_SCHED_DEBUG. So the ifdef should be
> something like:
> 
> #if defined(CONFIG_PREEMP_RT_BASE) && \
>     (defined(CONFIG_SMP) || \
>       (!defined(CONFIG_SMP) && defined(CONFIG_SCHED_DEBUG)))

That would still leave the issue with false positives if you have
CONFIG_DEBUG_PREEMPT but not CONFIG_SCHED_DEBUG (which should be the only
config currently experiencing build problems).

-Scott