Re: [RFC PATCH v1 09/13] mm: zswap: Config variable to enable compress batching in zswap_store().

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Oct 17, 2024 at 11:41 PM Kanchana P Sridhar
<kanchana.p.sridhar@intel.com> wrote:
>
> Add a new zswap config variable that controls whether zswap_store() will
> compress a batch of pages, for instance, the pages in a large folio:
>
>   CONFIG_ZSWAP_STORE_BATCHING_ENABLED
>
> The existing CONFIG_CRYPTO_DEV_IAA_CRYPTO variable added in commit
> ea7a5cbb4369 ("crypto: iaa - Add Intel IAA Compression Accelerator crypto
> driver core") is used to detect if the system has the Intel Analytics
> Accelerator (IAA), and the iaa_crypto module is available. If so, the
> kernel build will prompt for CONFIG_ZSWAP_STORE_BATCHING_ENABLED. Hence,
> users have the ability to set CONFIG_ZSWAP_STORE_BATCHING_ENABLED="y" only
> on systems that have Intel IAA.
>
> If CONFIG_ZSWAP_STORE_BATCHING_ENABLED is enabled, and IAA is configured
> as the zswap compressor, zswap_store() will process the pages in a large
> folio in batches, i.e., multiple pages at a time. Pages in a batch will be
> compressed in parallel in hardware, then stored. On systems without Intel
> IAA and/or if zswap uses software compressors, pages in the batch will be
> compressed sequentially and stored.
>
> The patch also implements a zswap API that returns the status of this
> config variable.

If we are compressing a large folio and batching is an option, is not
batching ever the correct thing to do? Why is the config option
needed?

>
> Suggested-by: Ying Huang <ying.huang@intel.com>
> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
> ---
>  include/linux/zswap.h |  6 ++++++
>  mm/Kconfig            | 12 ++++++++++++
>  mm/zswap.c            | 14 ++++++++++++++
>  3 files changed, 32 insertions(+)
>
> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> index d961ead91bf1..74ad2a24b309 100644
> --- a/include/linux/zswap.h
> +++ b/include/linux/zswap.h
> @@ -24,6 +24,7 @@ struct zswap_lruvec_state {
>         atomic_long_t nr_disk_swapins;
>  };
>
> +bool zswap_store_batching_enabled(void);
>  unsigned long zswap_total_pages(void);
>  bool zswap_store(struct folio *folio);
>  bool zswap_load(struct folio *folio);
> @@ -39,6 +40,11 @@ bool zswap_never_enabled(void);
>
>  struct zswap_lruvec_state {};
>
> +static inline bool zswap_store_batching_enabled(void)
> +{
> +       return false;
> +}
> +
>  static inline bool zswap_store(struct folio *folio)
>  {
>         return false;
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 33fa51d608dc..26d1a5cee471 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -125,6 +125,18 @@ config ZSWAP_COMPRESSOR_DEFAULT
>         default "zstd" if ZSWAP_COMPRESSOR_DEFAULT_ZSTD
>         default ""
>
> +config ZSWAP_STORE_BATCHING_ENABLED
> +       bool "Batching of zswap stores with Intel IAA"
> +       depends on ZSWAP && CRYPTO_DEV_IAA_CRYPTO
> +       default n
> +       help
> +       Enables zswap_store to swapout large folios in batches of 8 pages,
> +       rather than a page at a time, if the system has Intel IAA for hardware
> +       acceleration of compressions. If IAA is configured as the zswap
> +       compressor, this will parallelize batch compression of upto 8 pages
> +       in the folio in hardware, thereby improving large folio compression
> +       throughput and reducing swapout latency.
> +
>  choice
>         prompt "Default allocator"
>         depends on ZSWAP
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 948c9745ee57..4893302d8c34 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -127,6 +127,15 @@ static bool zswap_shrinker_enabled = IS_ENABLED(
>                 CONFIG_ZSWAP_SHRINKER_DEFAULT_ON);
>  module_param_named(shrinker_enabled, zswap_shrinker_enabled, bool, 0644);
>
> +/*
> + * Enable/disable batching of compressions if zswap_store is called with a
> + * large folio. If enabled, and if IAA is the zswap compressor, pages are
> + * compressed in parallel in batches of say, 8 pages.
> + * If not, every page is compressed sequentially.
> + */
> +static bool __zswap_store_batching_enabled = IS_ENABLED(
> +       CONFIG_ZSWAP_STORE_BATCHING_ENABLED);
> +
>  bool zswap_is_enabled(void)
>  {
>         return zswap_enabled;
> @@ -241,6 +250,11 @@ static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
>         pr_debug("%s pool %s/%s\n", msg, (p)->tfm_name,         \
>                  zpool_get_type((p)->zpool))
>
> +__always_inline bool zswap_store_batching_enabled(void)
> +{
> +       return __zswap_store_batching_enabled;
> +}
> +
>  /*********************************
>  * pool functions
>  **********************************/
> --
> 2.27.0
>