Subject: Re: [PATCH bpf-next 2/9] kthread: allow vararg kthread_{create,run}_on_cpu()

From: Stanislav Fomichev <[email-masked]>
Date: Fri, 30 Aug 2024 15:56:12 -0700

On 08/30, Alexander Lobakin wrote:
> Currently, kthread_{create,run}_on_cpu() doesn't support varargs like
> kthread_create{,_on_node}() do, which makes them less convenient to
> use.
> Convert them to take varargs as the last argument. The only difference
> is that they always append the CPU ID at the end and require the format
> string to have an excess '%u' at the end due to that. That's still true;
> meanwhile, the compiler will correctly point out to that if missing.
> One more nice side effect is that you can now use the underscored
> __kthread_create_on_cpu() if you want to override that rule and not
> have CPU ID at the end of the name.
> The current callers are not anyhow affected.
> 
> Signed-off-by: Alexander Lobakin <[email-masked]>
> ---
>  include/linux/kthread.h | 51 ++++++++++++++++++++++++++---------------
>  kernel/kthread.c        | 22 ++++++++++--------
>  2 files changed, 45 insertions(+), 28 deletions(-)
> 
> diff --git a/include/linux/kthread.h b/include/linux/kthread.h
> index b11f53c1ba2e..27a94e691948 100644
> --- a/include/linux/kthread.h
> +++ b/include/linux/kthread.h
> @@ -27,11 +27,21 @@ struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
>  #define kthread_create(threadfn, data, namefmt, arg...) \
>  	kthread_create_on_node(threadfn, data, NUMA_NO_NODE, namefmt, ##arg)
>  
> -
> -struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
> -					  void *data,
> -					  unsigned int cpu,
> -					  const char *namefmt);
> +__printf(4, 5)
> +struct task_struct *__kthread_create_on_cpu(int (*threadfn)(void *data),
> +					    void *data, unsigned int cpu,
> +					    const char *namefmt, ...);
> +
> +#define kthread_create_on_cpu(threadfn, data, cpu, namefmt, ...)	   \
> +	_kthread_create_on_cpu(threadfn, data, cpu, __UNIQUE_ID(cpu_),	   \
> +			       namefmt, ##__VA_ARGS__)
> +
> +#define _kthread_create_on_cpu(threadfn, data, cpu, uc, namefmt, ...) ({   \
> +	u32 uc = (cpu);							   \
> +									   \
> +	__kthread_create_on_cpu(threadfn, data, uc, namefmt,		   \
> +				##__VA_ARGS__, uc);			   \
> +})
>  
>  void get_kthread_comm(char *buf, size_t buf_size, struct task_struct *tsk);
>  bool set_kthread_struct(struct task_struct *p);
> @@ -62,25 +72,28 @@ bool kthread_is_per_cpu(struct task_struct *k);
>   * @threadfn: the function to run until signal_pending(current).
>   * @data: data ptr for @threadfn.
>   * @cpu: The cpu on which the thread should be bound,
> - * @namefmt: printf-style name for the thread. Format is restricted
> - *	     to "name.*%u". Code fills in cpu number.
> + * @namefmt: printf-style name for the thread. Must have an excess '%u'
> + *	     at the end as kthread_create_on_cpu() fills in CPU number.
>   *
>   * Description: Convenient wrapper for kthread_create_on_cpu()
>   * followed by wake_up_process().  Returns the kthread or
>   * ERR_PTR(-ENOMEM).
>   */
> -static inline struct task_struct *
> -kthread_run_on_cpu(int (*threadfn)(void *data), void *data,
> -			unsigned int cpu, const char *namefmt)
> -{
> -	struct task_struct *p;
> -
> -	p = kthread_create_on_cpu(threadfn, data, cpu, namefmt);
> -	if (!IS_ERR(p))
> -		wake_up_process(p);
> -
> -	return p;
> -}
> +#define kthread_run_on_cpu(threadfn, data, cpu, namefmt, ...)		   \
> +	_kthread_run_on_cpu(threadfn, data, cpu, __UNIQUE_ID(task_),	   \
> +			    namefmt, ##__VA_ARGS__)
> +
> +#define _kthread_run_on_cpu(threadfn, data, cpu, ut, namefmt, ...)	   \
> +({									   \
> +	struct task_struct *ut;						   \
> +									   \
> +	ut = kthread_create_on_cpu(threadfn, data, cpu, namefmt,	   \
> +				   ##__VA_ARGS__);			   \
> +	if (!IS_ERR(ut))						   \
> +		wake_up_process(ut);					   \
> +									   \
> +	ut;								   \
> +})

Why do you need to use __UNIQUE_ID here? Presumably ({}) in _kthread_run_on_cpu
should be enough to avoid the issue of non unique variable in the parent
scope. (and similar kthread_run isn't using any __UNIQUE_IDs)

The rest of the patches look good.