Re: [PATCH v4 10/17] preempt: Introduce HAS_SEPARATE_PREEMPT_RESCHED_BITS

[Boqun Feng <boqun@kernel.org>]

On Wed, Aug 05, 2026 at 02:39:20AM +0530, Shrikanth Hegde wrote:
> Hi.
> 
> > diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> > index 33fc4c814a9f..8299657f0f86 100644
> > --- a/include/linux/preempt.h
> > +++ b/include/linux/preempt.h
> > @@ -34,14 +34,31 @@
> >    *         SOFTIRQ_MASK:	0x0000ff00
> >    * HARDIRQ_DISABLE_MASK:	0x00ff0000
> >    *         HARDIRQ_MASK:	0x0f000000
> > + *
> > + * When HAS_SEPARATE_PREEMPT_RESCHED_BITS=y, PREEMPT_NEED_RESCHED is put in a
> > + * separate word and that allows 64bit load-store architectures to 'set'
> > + * PREEMPT_NEED_RESCHED without messing up the otherwise symmetric
> > + * modifications used on preempt_count and still load the whole thing
> > + * (single-copy) atomically, without having to resort to full atomic
> > + * operations.
> > + *
> > + * Because of the above, NMI_MASK bits are different depending on
> > + * HAS_SEPARATE_PREEMPT_RESCHED_BITS:
> > + *
> > + * - HAS_SEPARATE_PREEMPT_RESCHED_BITS=n:
> > + *
> >    *             NMI_MASK:	0x10000000
> >    * PREEMPT_NEED_RESCHED:	0x80000000
> > + *
> > + * - HAS_SEPARATE_PREEMPT_RESCHED_BITS=y:
> > + *             NMI_MASK:	0xf0000000
> > + * (PREEMPT_NEED_RESCHED is in a different word)
> >    */
> >   #define PREEMPT_BITS	8
> >   #define SOFTIRQ_BITS	8
> >   #define HARDIRQ_DISABLE_BITS	8
> >   #define HARDIRQ_BITS	4
> > -#define NMI_BITS	1
> > +#define NMI_BITS	(1 + 3*IS_ENABLED(CONFIG_HAS_SEPARATE_PREEMPT_RESCHED_BITS))
> 
> 
> Shouldn't testing/selftests/bpf/bpf_experimental.h also be updated with
> same?
> 

Yeah, sashiko found out the same thing.

> 
> >   #define PREEMPT_SHIFT	0
> >   #define SOFTIRQ_SHIFT	(PREEMPT_SHIFT + PREEMPT_BITS)
> > @@ -116,8 +133,8 @@ static __always_inline unsigned char interrupt_context_level(void)
> >    * preempt_count() is commonly implemented with READ_ONCE().
> >    */
> > -#define nmi_count()	(preempt_count() & NMI_MASK)
> > -#define hardirq_count()	(preempt_count() & HARDIRQ_MASK)
> > +#define nmi_count()		(preempt_count() & NMI_MASK)
> > +#define hardirq_count()		(preempt_count() & HARDIRQ_MASK)
> >   #ifdef CONFIG_PREEMPT_RT
> >   # define softirq_count()	(current->softirq_disable_cnt & SOFTIRQ_MASK)
> >   # define irq_count()		((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | softirq_count())
> > diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> > index 88c594c6d7fc..35f546a042b1 100644
> > --- a/kernel/Kconfig.preempt
> > +++ b/kernel/Kconfig.preempt
> > @@ -122,6 +122,10 @@ config PREEMPT_RT_NEEDS_BH_LOCK
> >   config PREEMPT_COUNT
> >          bool
> > +config HAS_SEPARATE_PREEMPT_RESCHED_BITS
> > +	bool
> > +	depends on PREEMPT_COUNT && 64BIT
> > +
> >   config PREEMPTION
> >          bool
> >          select PREEMPT_COUNT
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 9b3f1764fa9e..6d88343c3bad 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -5973,8 +5973,13 @@ void preempt_count_add(int val)
> >   #ifdef CONFIG_DEBUG_PREEMPT
> >   	/*
> >   	 * Underflow?
> > +	 *
> > +	 * Cannot detect underflow based on the current preempt_count() value
> > +	 * if using HAS_SEPARATE_PREEMPT_RESCHED_BITS because preempt count takes all 32
> > +	 * bits.
> >   	 */
> > -	if (DEBUG_LOCKS_WARN_ON((preempt_count() < 0)))
> > +	if (!IS_ENABLED(CONFIG_HAS_SEPARATE_PREEMPT_RESCHED_BITS) &&
> > +	    DEBUG_LOCKS_WARN_ON((preempt_count() < 0)))
> >   		return;
> >   #endif
> >   	__preempt_count_add(val);
> > @@ -6006,7 +6011,10 @@ void preempt_count_sub(int val)
> >   	/*
> >   	 * Underflow?
> >   	 */
> > -	if (DEBUG_LOCKS_WARN_ON(val > preempt_count()))
> > +	unsigned int uval = val;
> > +	unsigned int pc = preempt_count();
> > +
> > +	if (DEBUG_LOCKS_WARN_ON(pc - uval > pc))
> >   		return;
> >   	/*
> >   	 * Is the spinlock portion underflowing?
> > diff --git a/kernel/softirq.c b/kernel/softirq.c
> > index 0c9b2269a8d6..7980a4a232f9 100644
> > --- a/kernel/softirq.c
> > +++ b/kernel/softirq.c
> > @@ -103,7 +103,13 @@ void _local_interrupt_enable(void)
> >   }
> >   EXPORT_SYMBOL(_local_interrupt_enable);
> > +#ifndef CONFIG_HAS_SEPARATE_PREEMPT_RESCHED_BITS
> > +/*
> > + * Any 32bit architecture that still cares about performance should
> > + * probably ensure this is near preempt_count.
> > + */
> >   DEFINE_PER_CPU(unsigned int, nmi_nesting);
> > +#endif
> >   /*
> >    * SOFTIRQ_OFFSET usage:
> > diff --git a/lib/locking-selftest.c b/lib/locking-selftest.c
> > index bfafe1204c7b..c3d976c801bb 100644
> > --- a/lib/locking-selftest.c
> > +++ b/lib/locking-selftest.c
> > @@ -1429,7 +1429,7 @@ static int unexpected_testcase_failures;
> >   static void dotest(void (*testcase_fn)(void), int expected, int lockclass_mask)
> >   {
> > -	int saved_preempt_count = preempt_count();
> > +	long saved_preempt_count = preempt_count();
> 
> nit: Isn't preempt_count still returns int?
> 

Right, let me fix that. Thank you!

Regards,
Boqun

> >   #ifdef CONFIG_PREEMPT_RT
> >   	int saved_mgd_count = current->migration_disabled;
> >   	int saved_rcu_count = current->rcu_read_lock_nesting;
>