linux-kernel - Re: [PATCH] Fix isocpus's param handling when CPUMASK

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Message-ID: <20171023115050.twyx4kn7ttvon2ch@gmail.com>
Date:   Mon, 23 Oct 2017 13:50:50 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Rakib Mullick <rakib.mullick@...il.com>
Cc:     peterz@...radead.org, mka@...omium.org, longman@...hat.com,
        adobriyan@...il.com, tglx@...utronix.de, akpm@...ux-foundation.org,
        tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH]  Fix isocpus's param handling when CPUMASK_OFFSTACK=n.


* Rakib Mullick <rakib.mullick@...il.com> wrote:

> >  *On Fri, Oct 20, 2017 at 2:49 PM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> >> Rakib Mullick <rakib.mullick@...il.com> wrote:
> >>  include/linux/cpumask.h | 16 ++++++++++++++++
> >>  kernel/sched/topology.c |  8 +++++---
> >>  2 files changed, 21 insertions(+), 3 deletions(-)
> >
> > What kernel is this against? It does not apply to the latest kernels.
> 
> It was against 4.14-rc4, prepared before -rc5 release. Please, consider
> the below one, against -rc5.
> 
>  cpulist_parse() uses nr_cpumask_bits as limit to parse the
> passed buffer from kernel commandline. What nr_cpumask_bits
> represents varies depends upon CONFIG_CPUMASK_OFFSTACK option.
> If CONFIG_CPUMASK_OFFSTACK=n, then nr_cpumask_bits is same as
> NR_CPUS, which might not represent the # of cpus really exist
> (default 64). So, there's a chance of gap between nr_cpu_ids
> and NR_CPUS, which ultimately lead towards invalid cpulist_parse()
> operation. For example, if isolcpus=9 is passed on a 8 cpu
> system (CONFIG_CPUMASK_OFFSTACK=n) it doesn't show the error
> that it suppose to.
> 
> This patch fixes this issue by effectively find out the last
> cpu of the passed isolcpus list and checking it with nr_cpu_ids.
> Also, fixes the error message where the nr_cpu_ids should be
> nr_cpu_ids-1, since the cpu numbering starts from 0.
> 
> Signed-off-by: Rakib Mullick <rakib.mullick@...il.com>
> ---
>  include/linux/cpumask.h | 16 ++++++++++++++++
>  kernel/sched/topology.c |  8 +++++---
>  2 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index cd415b7..5631725 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -130,6 +130,11 @@ static inline unsigned int cpumask_first(const struct cpumask *srcp)
>  	return 0;
>  }
>  
> +static inline unsigned int cpumask_last(const struct cpumask *srcp)
> +{
> +	return 0;
> +}
> +
>  /* Valid inputs for n are -1 and 0. */
>  static inline unsigned int cpumask_next(int n, const struct cpumask *srcp)
>  {
> @@ -178,6 +183,17 @@ static inline unsigned int cpumask_first(const struct cpumask *srcp)
>  	return find_first_bit(cpumask_bits(srcp), nr_cpumask_bits);
>  }
>  
> +/**
> + * cpumask_last - get the last cpu in a cpumask

Please capitalize 'CPU' properly in documentation.

> + * @srcp:	- the cpumask pointer
> + *
> + * Returns	>= nr_cpumask_bits if no cpus set.
> + */
> +static inline unsigned int cpumask_last(const struct cpumask *srcp)
> +{
> +	return find_last_bit(cpumask_bits(srcp), nr_cpumask_bits);
> +}
> +
>  unsigned int cpumask_next(int n, const struct cpumask *srcp);
>  
>  /**
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index f1cf4f3..b9265c8 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -466,12 +466,14 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
>  /* Setup the mask of CPUs configured for isolated domains */
>  static int __init isolated_cpu_setup(char *str)
>  {
> -	int ret;
> +	int ret, lastcpu;
>  
>  	alloc_bootmem_cpumask_var(&cpu_isolated_map);
>  	ret = cpulist_parse(str, cpu_isolated_map);
> -	if (ret) {
> -		pr_err("sched: Error, all isolcpus= values must be between 0 and %u\n", nr_cpu_ids);
> +	lastcpu = cpumask_last(cpu_isolated_map);
> +	if (ret || lastcpu >= nr_cpu_ids) {

Any reason why 'lastcpu' has to be introduced - why not just use cpumask_last() 
directly in the condition? It looks obvious enough of a pattern.

> +		pr_err("sched: Error, all isolcpus= values must be between 0 and %u\n",
> +				nr_cpu_ids-1);

Please don't break the line mindlessly just due to checkpatch complaining - it 
makes the code less readable.

Thanks,

	Ingo