Re: [PATCH v5 1/2] kernfs: use hashed mutex and spinlock in place of global ones.

[Tejun Heo <tj@kernel.org>]

On Sun, Feb 06, 2022 at 12:09:24PM +1100, Imran Khan wrote:
> +/*
> + * NR_KERNFS_LOCK_BITS determines size (NR_KERNFS_LOCKS) of hash
> + * table of locks.
> + * Having a small hash table would impact scalability, since
> + * more and more kernfs_node objects will end up using same lock
> + * and having a very large hash table would waste memory.
> + *
> + * At the moment size of hash table of locks is being set based on
> + * the number of CPUs as follows:
> + *
> + * NR_CPU      NR_KERNFS_LOCK_BITS      NR_KERNFS_LOCKS
> + *   1                  1                       2
> + *  2-3                 2                       4
> + *  4-7                 4                       16
> + *  8-15                6                       64
> + *  16-31               8                       256
> + *  32 and more         10                      1024
> + */
> +#ifdef CONFIG_SMP
> +#define NR_KERNFS_LOCK_BITS (2 * (ilog2(NR_CPUS < 32 ? NR_CPUS : 32)))
> +#else
> +#define NR_KERNFS_LOCK_BITS     1
> +#endif
> +
> +#define NR_KERNFS_LOCKS     (1 << NR_KERNFS_LOCK_BITS)

I have a couple questions:

* How did you come up with the above numbers? Are they based on some
  experimentation? It'd be nice if the comment explains why the numbers are
  like that.

* IIRC, we split these locks to per kernfs instance recently as a way to
  mitigate lock contention occurring across kernfs instances. I don't think
  it's beneficial to keep these hashed locks separate. It'd be both simpler
  and less contended to double one shared hashtable than splitting the table
  into two separate half sized ones. So, maybe switch to global hashtables
  and increase the size?

Thanks.

-- 
tejun