Re: [PATCH v1 1/2] cpumask: Fix invalid uniprocessor mask assumption
From: Sander Vanheule
Date: Fri Jun 03 2022 - 11:14:01 EST
On Thu, 2022-06-02 at 15:48 -0700, Yury Norov wrote:
> On Thu, Jun 02, 2022 at 11:04:19PM +0200, Sander Vanheule wrote:
> > On uniprocessor builds, any CPU mask is assumed to contain exactly one
> > CPU (cpu0). This ignores the existence of empty masks, resulting in
> > incorrect behaviour for most of the implemented optimisations.
> >
> > Replace the uniprocessor optimisations with implementations that also
> > take into account empty masks. Also drop the incorrectly optimised
> > for_each_cpu implementations and use the generic implementations in all
> > cases.
> >
> > Signed-off-by: Sander Vanheule <sander@xxxxxxxxxxxxx>
> > ---
> > include/linux/cpumask.h | 35 +++++++++++++++--------------------
> > 1 file changed, 15 insertions(+), 20 deletions(-)
> >
> > diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> > index fe29ac7cc469..ce8c7b27f6c9 100644
> > --- a/include/linux/cpumask.h
> > +++ b/include/linux/cpumask.h
> > @@ -117,51 +117,54 @@ static __always_inline unsigned int
> > cpumask_check(unsigned int cpu)
> > }
> >
> > #if NR_CPUS == 1
> > -/* Uniprocessor. Assume all masks are "1". */
> > +/* Uniprocessor. Assume all valid masks are "1" or empty. */
> > static inline unsigned int cpumask_first(const struct cpumask *srcp)
> > {
> > - return 0;
> > + return !(*cpumask_bits(srcp) & 1);
> > }
>
> I think, you can just drop this '#if NR_CPUS == 1' part and rely on SMP
> versions. find_first_bit() is optimized for short bitmaps, so I expect
> no overhead comparing to this.
I was initially planning to just drop the UP versions, but that means I have to
include lib/cpumask.o on all builds. I'll have another look at this.
[...]
> >
> > static inline unsigned int cpumask_next_wrap(int n, const struct cpumask
> > *mask,
> > int start, bool wrap)
> > {
> > - /* cpu0 unless stop condition, wrap and at cpu0, then
> > nr_cpumask_bits */
> > - return (wrap && n == 0);
> > + /*
> > + * nr_cpumask_bits at stop condition, wrap and at cpu0, or empty
> > mask
> > + * otherwise cpu0
> > + */
> > + return (wrap && n == 0) || cpumask_first(mask);
> > }
My tests contained a copy-paste error, so I missed a case. The results currently
still differ from the SMP implementation though, so this looks like another
reason to just drop the UP optimised version.
> >
> > /* cpu must be a valid cpu, ie 0, so there's no other choice. */
> > @@ -186,14 +189,6 @@ static inline int cpumask_any_distribute(const struct
> > cpumask *srcp)
> > return cpumask_first(srcp);
> > }
> >
> > -#define for_each_cpu(cpu, mask) \
> > - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
> > -#define for_each_cpu_not(cpu, mask) \
> > - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
> > -#define for_each_cpu_wrap(cpu, mask, start) \
> > - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)(start))
> > -#define for_each_cpu_and(cpu, mask1, mask2) \
> > - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask1, (void)mask2)
> > #else
> > /**
> > * cpumask_first - get the first cpu in a cpumask
> > @@ -259,11 +254,13 @@ static inline unsigned int cpumask_next_zero(int n,
> > const struct cpumask *srcp)
> > }
> >
> > int __pure cpumask_next_and(int n, const struct cpumask *, const struct
> > cpumask *);
> > +extern int cpumask_next_wrap(int n, const struct cpumask *mask, int start,
> > bool wrap);
>
> is this extern needed?
I don't think it is, but this is a line that I just moved around in the header.
Like for the other functions, I could even add the __pure qualifier.
Thanks for the feedback,
Sander