On Sat, Apr 25, 2009 at 11:20:22AM +1000, npiggin@xxxxxxx wrote:
> Improve scalability of files_lock by adding per-cpu, per-sb files lists,
> protected with per-cpu locking. Effectively turning it into a big-writer
> lock.
Og dumb. Many locks. Many ifdefs. Og don't like.
> void file_sb_list_add(struct file *file, struct super_block *sb)
> {
> - spin_lock(&files_lock);
> + spinlock_t *lock;
> + struct list_head *list;
> + int cpu;
> +
> + lock = &get_cpu_var(files_cpulock);
> +#ifdef CONFIG_SMP
> + BUG_ON(file->f_sb_list_cpu != -1);
> + cpu = smp_processor_id();
> + list = per_cpu_ptr(sb->s_files, cpu);
> + file->f_sb_list_cpu = cpu;
> +#else
> + list = &sb->s_files;
> +#endif
> + spin_lock(lock);
> BUG_ON(!list_empty(&file->f_u.fu_list));
> - list_add(&file->f_u.fu_list, &sb->s_files);
> - spin_unlock(&files_lock);
> + list_add(&file->f_u.fu_list, list);
> + spin_unlock(lock);
> + put_cpu_var(files_cpulock);
> }
Don't like overhead on hot paths either.
And grown memory footprint of struct super_block (with alloc_percpu())
> atomic_long_t f_count;
> unsigned int f_flags;
> fmode_t f_mode;
> @@ -1330,7 +1333,11 @@ struct super_block {
> struct list_head s_io; /* parked for writeback */
> struct list_head s_more_io; /* parked for more writeback */
> struct hlist_head s_anon; /* anonymous dentries for (nfs) exporting */
> +#ifdef CONFIG_SMP
> + struct list_head *s_files;
> +#else
> struct list_head s_files;
> +#endif
> /* s_dentry_lru and s_nr_dentry_unused are protected by dcache_lock */
> struct list_head s_dentry_lru; /* unused dentry lru */
> int s_nr_dentry_unused; /* # of dentry on lru */
... and ifdefs like that in structs.
What I really want to see is a rationale for all that. Preferably with
more than microbenchmarks showing a visible impact.
Especially if you compare it with alternative variant that simply splits
files_lock on per-sb basis.
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