How to use sysfs inside kernel module?

In userspace I can just echo noop > /sys/block/sda/queue/scheduler.

How to do the same inside a kernel module?

I expect something like this (pseudocode):

struct dentry* e = sysfs_get_root();
vfs_path_lookup(e, ????, "block/sda/queue/scheduler", ???);
????;
struct something* q = ????;
????->store(q, "noop", 1);
/* some cleanup */

How to implement it properly?

My kernel module just registers SysRQ handler and should configure the io scheduler when that SysRQ is triggered 开发者_开发技巧(userspace programs can be hung at that time because of the bad io-scheduler)

There is just no way to implement it properly. If you want to do it anyway, and also understand the reason why it is a Bad Idea (tm), see this article

If you want to configure something for your kernel module, you can do that in a wrapper script which inserts your kernel module using insmod command.

And have a look at this article where it tell "Why it is bad to write files from Kernel"

Wrong wrong wrong. sysfs is an interface to userspace, you should not be using it inside the kernel.

If your module wants to change the block scheduler then you should work out how to do that inside the kernel, ie. when a user writes to /sys/block/sda/queue/scheduler some kernel code is run, you should be calling that code directly.

Having said that this seems like a Bad Idea, how will you handle multiple block devices for example?