Re: [PATCH v3] proc: faster open/read/close with "permanent" files

[Joe Perches <joe@perches.com>]

On Sat, 2020-02-22 at 23:15 +0300, Alexey Dobriyan wrote:
> Now that "struct proc_ops" exist we can start putting there stuff which
> could not fly with VFS "struct file_operations"...
> 
> Most of fs/proc/inode.c file is dedicated to make open/read/.../close reliable
> in the event of disappearing /proc entries which usually happens if module is
> getting removed. Files like /proc/cpuinfo which never disappear simply do not
> need such protection.
> 
> Save 2 atomic ops, 1 allocation, 1 free per open/read/close sequence for such
> "permanent" files.
> 
> Enable "permanent" flag for
> 
> 	/proc/cpuinfo
> 	/proc/kmsg
> 	/proc/modules
> 	/proc/slabinfo
> 	/proc/stat
> 	/proc/sysvipc/*
> 	/proc/swaps
> 
> More will come once I figure out foolproof way to prevent out module
> authors from marking their stuff "permanent" for performance reasons
> when it is not.
> 
> This should help with scalability: benchmark is "read /proc/cpuinfo R times
> by N threads scattered over the system".

Is this an actual expected use-case?
Is there some additional unnecessary memory consumption
in the unscaled systems?

And trivia:

>  static loff_t proc_reg_llseek(struct file *file, loff_t offset, int whence)
>  {
[]
> +	if (pde_is_permanent(pde)) {
> +		return pde_lseek(pde, file, offset, whence);
> +	} else if (use_pde(pde)) {
> +		rv = pde_lseek(pde, file, offset, whence);
>  		unuse_pde(pde);
>  	}
>  	return rv;
>  }
[]
>  static ssize_t proc_reg_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>  {
>  	struct proc_dir_entry *pde = PDE(file_inode(file));
>  	ssize_t rv = -EIO;
> -	if (use_pde(pde)) {
> -		typeof_member(struct proc_ops, proc_read) read;
>  
> -		read = pde->proc_ops->proc_read;
> -		if (read)
> -			rv = read(file, buf, count, ppos);
> +	if (pde_is_permanent(pde)) {
> +		return pde_read(pde, file, buf, count, ppos);
> +	} else if (use_pde(pde)) {
> +		rv = pde_read(pde, file, buf, count, ppos);
>  		unuse_pde(pde);

Perhaps all the function call duplication could be minimized
by using code without direct returns like:

	rv = pde_read(pde, file, buf, count, pos);
	if (!pde_is_permanent(pde))
		unuse_pde(pde);

	return rv;