Re: [PATCH 10/24] preempt: Introduce HAS_SEPARATE_PREEMPT_RESCHED_BITS

[Boqun Feng <boqun@kernel.org>]

On Mon, Aug 03, 2026 at 01:38:17PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 31, 2026 at 01:30:11PM -0700, Boqun Feng wrote:
> 
> > diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h
> > index 1220656f3370..12353eeebc52 100644
> > --- a/arch/x86/include/asm/preempt.h
> > +++ b/arch/x86/include/asm/preempt.h
> 
> > @@ -24,18 +34,26 @@ DECLARE_PER_CPU_CACHE_HOT(int, __preempt_count);
> >   */
> >  static __always_inline int preempt_count(void)
> >  {
> > -	return raw_cpu_read_4(__preempt_count) & ~PREEMPT_NEED_RESCHED;
> > +	return __pc_op(read, __preempt_count) & ~PREEMPT_NEED_RESCHED;
> >  }
> >  
> > -static __always_inline void preempt_count_set(int pc)
> > +/*
> > + * unsigned long preempt count parameter works for both 32bit and 64bit cases:
> > + *
> > + * - For 32bit, "int" (the return of preempt_count()) and "unsigned long" have
> > + *   the same size.
> > + * - For 64bit, the effective bits of a preempt count sits in 32bit, and we
> > + *   reserve the NEED_RESCHED bit from the old count.
> > + */
> 
> The 64bit comment doesn't really make sense to me.
> 

Ah, I meant "preserve" instead of "reserve"..

+/*
+ * unsigned long preempt count parameter works for both 32bit and 64bit cases:
+ *
+ * - For 32bit, "int" (the return of preempt_count()) and "unsigned long" have
+ *   the same size.
+ * - For 64bit, the effective bits of a preempt count sits in 32bit, and we
+ *   preserve the NEED_RESCHED bit from the old count.
+ */

It means we won't change the NEED_RESCHED bit regardless that `pc` has
it set or not.

> > +static __always_inline void preempt_count_set(unsigned long pc)
> >  {
> > -	int old, new;
> > +	unsigned long old, new;
> >  
> > -	old = raw_cpu_read_4(__preempt_count);
> > +	old = __pc_op(read, __preempt_count);
> >  	do {
> >  		new = (old & PREEMPT_NEED_RESCHED) |
> >  			(pc & ~PREEMPT_NEED_RESCHED);
> > -	} while (!raw_cpu_try_cmpxchg_4(__preempt_count, &old, new));
> > +	} while (!__pc_op(try_cmpxchg, __preempt_count, &old, new));
> >  }
> 
> > diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> > index 33fc4c814a9f..87d5367f986c 100644
> > --- a/include/linux/preempt.h
> > +++ b/include/linux/preempt.h
> > @@ -30,18 +30,20 @@
> >   * NMI nesting depth is tracked in a separate per-CPU variable
> >   * (nmi_nesting) to save bits in preempt_count.
> >   *
> > - *         PREEMPT_MASK:	0x000000ff
> > - *         SOFTIRQ_MASK:	0x0000ff00
> > - * HARDIRQ_DISABLE_MASK:	0x00ff0000
> > - *         HARDIRQ_MASK:	0x0f000000
> > - *             NMI_MASK:	0x10000000
> > - * PREEMPT_NEED_RESCHED:	0x80000000
> > + *				32bit		HAS_SEPARATE_PREEMPT_RESCHED_BITS
> > + *
> > + *         PREEMPT_MASK:	0x000000ff	0x00000000000000ff
> > + *         SOFTIRQ_MASK:	0x0000ff00	0x000000000000ff00
> > + * HARDIRQ_DISABLE_MASK:	0x00ff0000	0x0000000000ff0000
> > + *         HARDIRQ_MASK:	0x0f000000	0x000000000f000000
> > + *             NMI_MASK:	0x10000000	0x00000000f0000000
> > + * PREEMPT_NEED_RESCHED:	0x80000000	0x8000000000000000
> >   */
> 
> Perhaps add a comment about how HAS_SEPARATE_PREEMPT_RESCHED_BITS really
> is about having PREEMPT_NEED_RESCHED in its own word, rather than
> preempt_count() being 64bit.
> 
> Because as presented it is very easy to confuse these two options.
> Ideally it would explain the LOAD-STORE issue with NEED_RESCHED and
> point to ARM64 or something.

How about organizing the comments as following:

+ *
+ *         PREEMPT_MASK:	0x000000ff
+ *         SOFTIRQ_MASK:	0x0000ff00
+ * HARDIRQ_DISABLE_MASK:	0x00ff0000
+ *         HARDIRQ_MASK:	0x0f000000
+ *
+ * Depending on HAS_SEPARATE_PREEMPT_RESCHED_BITS, NEED_RESCHED bit
+ * is put in a separate 32bits.
+ *
+ * HAS_SEPARATE_PREEMPT_RESCHED_BITS=n:
+ *
+ *             NMI_MASK:	0x10000000
+ * PREEMPT_NEED_RESCHED:	0x80000000
+ *
+ * HAS_SEPARATE_PREEMPT_RESCHED_BITS=y:
+ *
+ * (NMI_MASK can use all the 4 bits)
+ *
+ *             NMI_MASK:	0xf0000000

Thoughts?

Regards,
Boqun