Message-ID: <Ypk+MKSSaSCBnITY@yury-laptop>
Date:   Thu, 2 Jun 2022 15:48:16 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Sander Vanheule <sander@...nheule.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Marco Elver <elver@...gle.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v1 1/2] cpumask: Fix invalid uniprocessor mask assumption

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@...nheule.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.
  
>  static inline unsigned int cpumask_first_zero(const struct cpumask *srcp)
>  {
> -	return 0;
> +	return *cpumask_bits(srcp) & 1;
>  }
>  
>  static inline unsigned int cpumask_first_and(const struct cpumask *srcp1,
>  					     const struct cpumask *srcp2)
>  {
> -	return 0;
> +	return !(*cpumask_bits(srcp1) & *cpumask_bits(srcp2) & 1);
>  }
>  
>  static inline unsigned int cpumask_last(const struct cpumask *srcp)
>  {
> -	return 0;
> +	return cpumask_first(srcp);
>  }
>  
>  /* Valid inputs for n are -1 and 0. */
>  static inline unsigned int cpumask_next(int n, const struct cpumask *srcp)
>  {
> -	return n+1;
> +	return !!(n + 1 + cpumask_first(srcp));
>  }
>  
>  static inline unsigned int cpumask_next_zero(int n, const struct cpumask *srcp)
>  {
> -	return n+1;
> +	return !!(n + 1 + cpumask_first_zero(srcp));
>  }
>  
>  static inline unsigned int cpumask_next_and(int n,
>  					    const struct cpumask *srcp,
>  					    const struct cpumask *andp)
>  {
> -	return n+1;
> +	return !!(n + 1 + cpumask_first_and(srcp, andp));
>  }
>  
>  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);
>  }
>  
>  /* 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?

Thanks,
Yury

>  int __pure cpumask_any_but(const struct cpumask *mask, unsigned int cpu);
>  unsigned int cpumask_local_spread(unsigned int i, int node);
>  int cpumask_any_and_distribute(const struct cpumask *src1p,
>  			       const struct cpumask *src2p);
>  int cpumask_any_distribute(const struct cpumask *srcp);
> +#endif /* SMP */
>  
>  /**
>   * for_each_cpu - iterate over every cpu in a mask
> @@ -289,7 +286,6 @@ int cpumask_any_distribute(const struct cpumask *srcp);
>  		(cpu) = cpumask_next_zero((cpu), (mask)),	\
>  		(cpu) < nr_cpu_ids;)
>  
> -extern int cpumask_next_wrap(int n, const struct cpumask *mask, int start, bool wrap);
>  
>  /**
>   * for_each_cpu_wrap - iterate over every cpu in a mask, starting at a specified location
> @@ -324,7 +320,6 @@ extern int cpumask_next_wrap(int n, const struct cpumask *mask, int start, bool
>  	for ((cpu) = -1;						\
>  		(cpu) = cpumask_next_and((cpu), (mask1), (mask2)),	\
>  		(cpu) < nr_cpu_ids;)
> -#endif /* SMP */
>  
>  #define CPU_BITS_NONE						\
>  {								\
> -- 
> 2.36.1

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