Messages in this thread | | | From | Waiman Long <> | Subject | Re: [patch 04/50] sched: Rework the __schedule() preempt argument | Date | Tue, 13 Jul 2021 16:04:03 -0400 |
| |
On 7/13/21 11:10 AM, Thomas Gleixner wrote: > From: Thomas Gleixner <tglx@linutronix.de> > > PREEMPT_RT needs to hand a special state into __schedule() when a task > blocks on a 'sleeping' spin/rwlock. This is required to handle > rcu_note_context_switch() correctly without having special casing in the > RCU code. From an RCU point of view the blocking on the sleeping spinlock > is equivalent to preemption because the task might be in a read side > critical section. > > schedule_debug() also has a check which would trigger with the !preempt > case, but that could be handled differently. > > To avoid adding another argument and extra checks which cannot be optimized > out by the compiler the following solution has been chosen: > > - Replace the boolean 'preempt' argument with an unsigned integer > 'sched_mode' argument and define constants to hand in: > (0 == No preemption, 1 = preemption). > > - Add two masks to apply on that mode one for the debug/rcu invocations > and one for the actual scheduling decision. > > For a non RT kernel these masks are UINT_MAX, i.e. all bits are set > which allows the compiler to optimze the AND operation out because it is > not masking out anything. IOW, it's not different from the boolean. > > RT enabled kernels will define these masks seperately. > > No functional change. > > Signed-off-by: Thomas Gleixner <tglx@linutronix.de> > --- > kernel/sched/core.c | 36 +++++++++++++++++++++++++----------- > 1 file changed, 25 insertions(+), 11 deletions(-) > --- > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -5822,6 +5822,20 @@ pick_next_task(struct rq *rq, struct tas > #endif /* CONFIG_SCHED_CORE */ > > /* > + * Constants for the sched_mode argument of __schedule(). > + * > + * The mode argument allows RT enabled kernels to differentiate a > + * preemption from blocking on an 'sleeping' spin/rwlock by having seperate > + * mask values for SM_MASK_PREEMPT and SM_MASK_STATE while on a non RT > + * enabled kernel the masks have all bits set which allows the compiler to > + * optimize the AND operation out. > + */ > +#define SM_NONE 0x0 > +#define SM_PREEMPT 0x1 > +#define SM_MASK_PREEMPT UINT_MAX > +#define SM_MASK_STATE SM_MASK_PREEMPT
Nit: Inconsistent use of space/tab after #define.
Cheers, Longman
| |