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

Joe Perches
2020-02-22T20:41:09Z

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;