[PATCH 4/4] workqueue: slash half memory usage in 32bit system

[Lai Jiangshan]

The major memory ussage in workqueue is on the pool_workqueue.
The pool_workqueue has alignment requirement which often leads
to padding.

Reducing the memory usage for the pool_workqueue is valuable.

And 32bit system often has less memory, less workqueues,
less works, less concurrent flush_workqueue()s, so we can
slash the flush color on 32bit system to reduce memory usage

Before patch:
The sizeof the struct pool_workqueue is 256 bytes,
only 136 bytes is in use in 32bit system

After patch:
The sizeof the struct pool_workqueue is 128 bytes,
only 104 bytes is in use in 32bit system, there is still
room for future usage.

Setting WORK_STRUCT_COLOR_BITS to 3 can't reduce the sizeof
the struct pool_workqueue in 64bit system, unless combined
with big refactor for unbound pwq.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
 include/linux/workqueue.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 26de0cae2a0a..c0f311926d01 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -39,7 +39,11 @@ enum {
 	WORK_STRUCT_COLOR_SHIFT	= 4,	/* color for workqueue flushing */
 #endif
 
+#if BITS_PER_LONG == 32
+	WORK_STRUCT_COLOR_BITS	= 3,
+#else
 	WORK_STRUCT_COLOR_BITS	= 4,
+#endif
 
 	WORK_STRUCT_PENDING	= 1 << WORK_STRUCT_PENDING_BIT,
 	WORK_STRUCT_DELAYED	= 1 << WORK_STRUCT_DELAYED_BIT,
@@ -65,6 +69,8 @@ enum {
 	 * Reserve 8 bits off of pwq pointer w/ debugobjects turned off.
 	 * This makes pwqs aligned to 256 bytes and allows 15 workqueue
 	 * flush colors.
+	 * For 32 bit system, the numbers are 7 bits, 128 bytes, 7 colors
+	 * respectively.
 	 */
 	WORK_STRUCT_FLAG_BITS	= WORK_STRUCT_COLOR_SHIFT +
 				  WORK_STRUCT_COLOR_BITS,
-- 
2.20.1