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Subject[PATCH 0/7] Add rcu_sync infrastructure to avoid _expedited() in percpu-rwsem

Let me make another attempt to push rcu_sync and add a _simple_
improvment into percpu-rwsem. It already has another user (cgroups)
and I think it can have more. Peter has some use-cases. sb->s_writers
(which afaics is buggy btw) can be turned into percpu-rwsem too I think.

Linus, I am mostly trying to convince you. Nobody else objected so far.
Could you please comment?

Peter, if you agree with 5-7, can I add your Signed-off-by's ?

To me, the most annoying problem with percpu_rw_semaphore is
synchronize_sched_expedited() which is called twice by every
down_write/up_write. I think it would be really nice to avoid it.

Let's start with the simple test-case,


perf probe -x /lib/ syscall

for i in {1..1000}; do
echo 1 >| /sys/kernel/debug/tracing/events/probe_libc/syscall/enable
echo 0 >| /sys/kernel/debug/tracing/events/probe_libc/syscall/enable

It needs ~ 13.5 seconds (2 CPUs, KVM). If we simply replace
synchronize_sched_expedited() with synchronize_sched() it takes
~ 67.5 seconds. This is not good.

With these patches it takes around 13.3 seconds again (a little
bit faster), and it doesn't use _expedited. synchronize_sched()
is called 1-2 (max 3) times in average. And now it does not
disturb the whole system.

And just in case, I also measured

for (i = 0; i < 1000000; ++i) {

and it runs more than 1.5 times faster (to remind, only 2 CPUs),
but this is not that interesting, I agree.

And note that the actual change in percpu-rwsem is really simple,
and imo it even makes the code simpler. (the last patch is off-
topic cleanup).

So the only complication is rcu_sync itself. But, rightly or not (I
am obviously biased), I believe this new rcu infrastructure is natural
and useful, and I think it can have more users too.

And. We can do more improvements in rcu_sync and percpu-rwsem, and
I don't only mean other optimizations from Peter. In particular, we
can extract the "wait for gp pass" from rcu_sync_enter() into another
helper, we can teach percpu_down_write() to allow multiple writers,
and more.


include/linux/percpu-rwsem.h | 3 +-
include/linux/rcusync.h | 57 +++++++++++++++
kernel/locking/percpu-rwsem.c | 78 ++++++---------------
kernel/rcu/Makefile | 2 +-
kernel/rcu/sync.c | 152 +++++++++++++++++++++++++++++++++++++++++
5 files changed, 235 insertions(+), 57 deletions(-)

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