[PATCH v2 12/13] mm: Add sysctl vm.compress-batching switch for compress batching during swapout.

[Kanchana P Sridhar <kanchana.p.sridhar@intel.com>]

The sysctl vm.compress-batching parameter is 0 by default. If the platform
has Intel IAA, the user can run experiments with IAA compress batching of
large folios in zswap_store() as follows:

sysctl vm.compress-batching=1
echo deflate-iaa > /sys/module/zswap/parameters/compressor

This is expected to significantly improve zswap_store() latency of swapping
out large folios due to parallel compression of 8 pages in the large folio
at a time, in hardware.

Setting vm.compress-batching to "1" takes effect only if the zswap
compression algorithm's crypto_acomp registers implementations for the
batch_compress() and batch_decompress() API. In other words, compress
batching works only with the iaa_crypto driver, that does register these
new batching API. It is a no-op for compressors that do not register the
batching API.

The sysctl vm.compress-batching acts as a switch because it takes effect
upon future zswap_store() calls on any given core. If the switch is "1",
large folios will use parallel batched compression of the folio's pages.
If the switch is "0", zswap_store() will use sequential compression for
storing every page in a large folio.

Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
---
 include/linux/mm.h | 2 ++
 kernel/sysctl.c    | 9 +++++++++
 mm/swap.c          | 6 ++++++
 3 files changed, 17 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 610653a8b0fe..c94ba5c36169 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -82,8 +82,10 @@ extern const int page_cluster_max;
 
 #ifdef CONFIG_SYSCTL
 extern int sysctl_legacy_va_layout;
+extern unsigned int compress_batching;
 #else
 #define sysctl_legacy_va_layout 0
+#define compress_batching 0
 #endif
 
 #ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 79e6cb1d5c48..e298857595b4 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2064,6 +2064,15 @@ static struct ctl_table vm_table[] = {
 		.extra1		= SYSCTL_ZERO,
 		.extra2		= (void *)&page_cluster_max,
 	},
+	{
+		.procname	= "compress-batching",
+		.data		= &compress_batching,
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0644,
+		.proc_handler	= proc_douintvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
+	},
 	{
 		.procname	= "dirtytime_expire_seconds",
 		.data		= &dirtytime_expire_interval,
diff --git a/mm/swap.c b/mm/swap.c
index 638a3f001676..bc4c9079769e 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -47,6 +47,9 @@
 int page_cluster;
 const int page_cluster_max = 31;
 
+/* Enable/disable compress batching during swapout. */
+unsigned int compress_batching;
+
 struct cpu_fbatches {
 	/*
 	 * The following folio batches are grouped together because they are protected
@@ -1074,4 +1077,7 @@ void __init swap_setup(void)
 	 * Right now other parts of the system means that we
 	 * _really_ don't want to cluster much more
 	 */
+
+	/* Disable compress batching during swapout by default. */
+	compress_batching = 0;
 }
-- 
2.27.0