On Wed 24-06-20 16:32:28, Shaokun Zhang wrote:
> get_file_rcu_many, which is called by __fget_files, has used
> atomic_try_cmpxchg now and it can reduce the access number of the global
> variable to improve the performance of atomic instruction compared with
> atomic_cmpxchg.
>
> __fget_files does check the @f_mode with mask variable and will do some
> atomic operations on @f_count, but both are on the same cacheline.
> Many CPU cores do file access and it will cause much conflicts on @f_count.
> If we could make the two members into different cachelines, it shall relax
> the siutations.
<snip nice unixbench results>
Thanks for the patch! The wins for your microbenchmark heavily sharing
struct file are nice but I'm not sure your change is a universal win. When
struct file is not shared (which is far more common), hot code paths like
__fget() or __fget_light() will now need to fetch two cache lines from
struct file instead of one. So I don't think that for most users the
tradeoff is really worth it...
Honza
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 3f881a892ea7..0faeab5622fb 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -955,7 +955,6 @@ struct file {
> */
> spinlock_t f_lock;
> enum rw_hint f_write_hint;
> - atomic_long_t f_count;
> unsigned int f_flags;
> fmode_t f_mode;
> struct mutex f_pos_lock;
> @@ -979,6 +978,7 @@ struct file {
> struct address_space *f_mapping;
> errseq_t f_wb_err;
> errseq_t f_sb_err; /* for syncfs */
> + atomic_long_t f_count;
> } __randomize_layout
> __attribute__((aligned(4))); /* lest something weird decides that 2 is OK */
>
> --
> 2.7.4
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
