Messages in this thread | | | Subject | Re: [PATCH v1 1/2] cpumask: Fix invalid uniprocessor mask assumption | From | Sander Vanheule <> | Date | Fri, 03 Jun 2022 17:13:52 +0200 |
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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@svanheule.net> > > --- > > 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
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