Re: [PATCH 4/4] mm, swap_cgroup: remove global swap cgroup lock

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Dec 2, 2024 at 10:42 AM Kairui Song <ryncsn@gmail.com> wrote:
>
> From: Kairui Song <kasong@tencent.com>
>
> commit e9e58a4ec3b1 ("memcg: avoid use cmpxchg in swap cgroup maintainance")
> replaced the cmpxchg/xchg with a global irq spinlock because some archs
> doesn't support 2 bytes cmpxchg/xchg. Clearly this won't scale well.
>
> And as commented in swap_cgroup.c, this lock is not needed for map
> synchronization.
>
> Emulation of 2 bytes cmpxchg/xchg with atomic isn't hard, so implement
> it to get rid of this lock.
>
> Testing using 64G brd and build with build kernel with make -j96 in 1.5G
> memory cgroup using 4k folios showed below improvement (10 test run):
>
> Before this series:
> Sys time: 10730.08 (stdev 49.030728)
> Real time: 171.03 (stdev 0.850355)
>
> After this commit:
> Sys time: 9612.24 (stdev 66.310789), -10.42%
> Real time: 159.78 (stdev 0.577193), -6.57%
>
> With 64k folios and 2G memcg:
> Before this series:
> Sys time: 7626.77 (stdev 43.545517)
> Real time: 136.22 (stdev 1.265544)
>
> After this commit:
> Sys time: 6936.03 (stdev 39.996280), -9.06%
> Real time: 129.65 (stdev 0.880039), -4.82%
>
> Sequential swapout of 8G 4k zero folios (24 test run):
> Before this series:
> 5461409.12 us (stdev 183957.827084)
>
> After this commit:
> 5420447.26 us (stdev 196419.240317)
>
> Sequential swapin of 8G 4k zero folios (24 test run):
> Before this series:
> 19736958.916667 us (stdev 189027.246676)
>
> After this commit:
> 19662182.629630 us (stdev 172717.640614)
>
> Performance is better or at least not worse for all tests above.
>
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  mm/swap_cgroup.c | 56 +++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 41 insertions(+), 15 deletions(-)
>
> diff --git a/mm/swap_cgroup.c b/mm/swap_cgroup.c
> index a76afdc3666a..028f5e6be3f0 100644
> --- a/mm/swap_cgroup.c
> +++ b/mm/swap_cgroup.c
> @@ -5,6 +5,15 @@
>
>  #include <linux/swapops.h> /* depends on mm.h include */
>
> +#define ID_PER_UNIT (sizeof(atomic_t) / sizeof(unsigned short))
> +struct swap_cgroup_unit {
> +       union {
> +               int raw;
> +               atomic_t val;
> +               unsigned short __id[ID_PER_UNIT];
> +       };
> +};
> +
>  static DEFINE_MUTEX(swap_cgroup_mutex);
>
>  struct swap_cgroup {
> @@ -12,8 +21,10 @@ struct swap_cgroup {
>  };
>
>  struct swap_cgroup_ctrl {
> -       unsigned short  *map;
> -       spinlock_t      lock;
> +       union {
> +               struct swap_cgroup_unit *units;
> +               unsigned short *map;
> +       };
>  };
>
>  static struct swap_cgroup_ctrl swap_cgroup_ctrl[MAX_SWAPFILES];
> @@ -31,6 +42,24 @@ static struct swap_cgroup_ctrl swap_cgroup_ctrl[MAX_SWAPFILES];
>   *
>   * TODO: we can push these buffers out to HIGHMEM.
>   */

While you're at it, I think the comment above is quite outdated :)