[RFC,0/19] Further shrink srcu_struct to promote cache locality

Message

Paul E. McKenney March 24, 2023, 12:19 a.m. UTC

Hello!

This RFC series shrinks the srcu_struct structure to the bare minimum
required to support SRCU readers, relegating the remaining fields to a new
srcu_usage structure.  Statically allocated srcu_struct structures created
by DEFINE_SRCU() and DEFINE_STATIC_SRCU() have statically allocated
srcu_usage structures, but those required for dynamically allocated
srcu_struct structures that are initialized using init_srcu_struct()
are dynamically allocated.

The results is a reduction in the size of an srcu_struct structure from
a couple hundred bytes to just 24 bytes on x86_64 systems.  This can be
helpful when SRCU readers are used in a fastpath for which the srcu_struct
structure must be embedded in another structure, and especially where
that fastpath also needs to access fields both before and after the
srcu_struct structure.

This series takes baby steps, in part because breaking SRCU means that
you get absolutely no console output.  Yes, I did learn this the hard way.
Why do you ask?  ;-)

Here are those baby steps:

1.	Add whitespace to __SRCU_STRUCT_INIT() & __DEFINE_SRCU().

2.	Use static init for statically allocated in-module srcu_struct.

3.	Begin offloading srcu_struct fields to srcu_update.  Note that
	this affects notifiers, which open-code static allocation of
	an srcu_struct structure.  (And no, I still do not see a way to
	abstract this, sorry!)

4.	Move ->level from srcu_struct to srcu_usage.

5.	Move ->srcu_size_state from srcu_struct to srcu_usage.

6.	Move ->srcu_cb_mutex from srcu_struct to srcu_usage.

7.	Move ->lock initialization after srcu_usage allocation.

8.	Move ->lock from srcu_struct to srcu_usage.

9.	Move ->srcu_gp_mutex from srcu_struct to srcu_usage.

10.	Move grace-period fields from srcu_struct to srcu_usage.

11.	Move heuristics fields from srcu_struct to srcu_usage.

12.	Move ->sda_is_static from srcu_struct to srcu_usage.

13.	Move srcu_barrier() fields from srcu_struct to srcu_usage.

14.	Move work-scheduling fields from srcu_struct to srcu_usage.

15.	Fix long lines in srcu_get_delay().

16.	Fix long lines in cleanup_srcu_struct().

17.	Fix long lines in srcu_gp_end().

18.	Fix long lines in srcu_funnel_gp_start().

19.	Remove extraneous parentheses from srcu_read_lock() etc.

						Thanx, Paul

------------------------------------------------------------------------

 b/include/linux/notifier.h |    5 
 b/include/linux/srcu.h     |    8 
 b/include/linux/srcutiny.h |    6 
 b/include/linux/srcutree.h |   28 +-
 b/kernel/rcu/rcu.h         |    6 
 b/kernel/rcu/srcutree.c    |   19 +
 include/linux/srcutree.h   |  123 ++++++-----
 kernel/rcu/srcutree.c      |  488 +++++++++++++++++++++++----------------------
 8 files changed, 368 insertions(+), 315 deletions(-)