On Sat, Nov 02, 2019 at 12:45:52PM +0000, Lai Jiangshan wrote: > Rather than using the generic version of preempt_count, x86 uses > a special version of preempt_count implementation that take advantage > of x86 (single instruction to access/add/decl/flip-bits of percpu > counter). It makes the preempt_count operations really cheap. > > For x86, rcu_preempt_depth can also take the same advantage by > using the same technique. > > After the patchset: > - No function call when using rcu_read_[un]lock(). > It is minor improvement, other arch can also achieve it via > moving ->rcu_read_lock_nesting and ->rcu_read_unlock_special > to thread_info, but inlined rcu_read_[un]lock() generates > more instructions and footprint in other arch generally. > - Only single instruction for rcu_read_lock(). > - Only 2 instructions for the fast path of rcu_read_unlock(). > > Patch4 simplifies rcu_read_unlock() by avoid using negative > ->rcu_read_lock_nesting, Patch7 introduces the percpu rcu_preempt_depth. > Other patches are for preparation. > > changed from v1: > drop patch1/2 of the v1 > drop merged patches > > Using special.b.deferred_qs to avoid wakeup in v1 is changed to using > preempt_count. And special.b.deferred_qs is removed. > > Lai Jiangshan (7): > rcu: use preempt_count to test whether scheduler locks is held > rcu: cleanup rcu_preempt_deferred_qs() > rcu: remove useless special.b.deferred_qs > rcu: don't use negative ->rcu_read_lock_nesting > rcu: wrap usages of rcu_read_lock_nesting > rcu: clear the special.b.need_qs in rcu_note_context_switch() > x86,rcu: use percpu rcu_preempt_depth I am getting on a plane shortly, so what I have done is start a longish rcutorture test on these. I will take a look at them later on. And I presume that answers to my questions on some of the patches in this series will be forthcoming. ;-) Thanx, Paul > arch/x86/Kconfig | 2 + > arch/x86/include/asm/rcu_preempt_depth.h | 87 +++++++++++++++++++ > arch/x86/kernel/cpu/common.c | 7 ++ > arch/x86/kernel/process_32.c | 2 + > arch/x86/kernel/process_64.c | 2 + > include/linux/rcupdate.h | 24 ++++++ > include/linux/sched.h | 2 +- > init/init_task.c | 2 +- > kernel/fork.c | 2 +- > kernel/rcu/Kconfig | 3 + > kernel/rcu/tree_exp.h | 35 ++------ > kernel/rcu/tree_plugin.h | 101 ++++++++++++++--------- > 12 files changed, 196 insertions(+), 73 deletions(-) > create mode 100644 arch/x86/include/asm/rcu_preempt_depth.h > > -- > 2.20.1 >
