Re: [PATCH v9 00/13] rcu: call_rcu() power improvements

From: Paul E. McKenney
Date: Mon Oct 17 2022 - 09:37:49 EST


On Sun, Oct 16, 2022 at 04:22:52PM +0000, Joel Fernandes (Google) wrote:
> v9 version of RCU lazy patches based on rcu/next branch.
> Only change since v8 is this discussion:
> https://lore.kernel.org/rcu/20221011180142.2742289-1-joel@xxxxxxxxxxxxxxxxx/T/#m8eff15110477f3430b3b02561b66f7b0d34a73b0
>
> To facilitate easier merge, I dropped tracing and other patches and just
> implemented the new changes. I will post the tracing patches later along with
> rcutop as I need to add new tracepoints that Frederic suggested.
>
> Main recent changes:
> 1. rcu_barrier() wake up only for lazy bypass list.
> 2. Make all call_rcu() default-lazy and add call_rcu_flush() API.
> 3. Take care of some callers using call_rcu_flush() API.
> 4. Several refactorings suggested by Paul/Frederic.
> 5. New call_rcu() to call_rcu_flush() conversions by Joel/Vlad/Paul.
>
> I am seeing good performance and power with these patches on real ChromeOS x86
> asymmetric hardware.
>
> Earlier cover letter with lots of details is here:
> https://lore.kernel.org/all/20220901221720.1105021-1-joel@xxxxxxxxxxxxxxxxx/
>
> List of recent changes:
>
> [ Frederic Weisbec: Program the lazy timer only if WAKE_NOT, since other
> deferral levels wake much earlier so for those it is not needed. ]
>
> [ Frederic Weisbec: Use flush flags to keep bypass API code clean. ]
>
> [ Frederic Weisbec: Make rcu_barrier() wake up only if main list empty. ]
>
> [ Frederic Weisbec: Remove extra 'else if' branch in rcu_nocb_try_bypass(). ]
>
> [ Joel: Fix issue where I was not resetting lazy_len after moving it to rdp ]
>
> [ Paul/Thomas/Joel: Make call_rcu() default lazy so users don't mess up. ]
>
> [ Paul/Frederic : Cosmetic changes, split out wakeup of nocb thread. ]
>
> [ Vlad/Joel : More call_rcu -> flush conversions ]
>
> [ debug code for detecting "wake" in kernel's call_rcu() callbacks. ]
>
> The following 2 scripts can be used to check if any callbacks in the kernel are
> doing a wake up (it is best effort and may miss some things, but we found
> issues using it)
>
> 1. Script to search for call_rcu() references and dump the callback list to a file:
> #!/bin/bash
>
> rm func-list
> touch func-list
>
> for f in $(find . \( -name "*.c" -o -name "*.h" \) | grep -v rcu); do
>
> funcs=$(perl -0777 -ne 'while(m/call_rcu\([&]?.+,\s?(.+)\).*;/g){print "$1\n";}' $f)
>
> if [ "x$funcs" != "x" ]; then
> for func in $funcs; do
> echo "$f $func" >> func-list
> echo "$f $func"
> done
> fi
>
> done
>
> cat func-list | sort | uniq | tee func-list-sorted
>
> 2. Script to search "wake" after callback references:
>
> #!/bin/bash
>
> while read fl; do
> file=$(echo $fl | cut -d " " -f1)
> func=$(echo $fl | cut -d " " -f2)
>
> grep -A 30 $func $file | grep wake > /dev/null
>
> if [ $? -eq 0 ]; then
> echo "keyword wake found after function reference $func in $file"
> echo "Output:"
> grep -A 30 $func $file
> echo "==========================================================="
> fi
> done < func-list-sorted

Very good, thank you all!

I have pulled these in for further review and testing.

I am holding off on the last one ("rcu/debug: Add wake-up debugging for
lazy callbacks") for the immediate future, but let's see how it goes.

Thanx, Paul

> Frederic Weisbecker (1):
> rcu: Fix missing nocb gp wake on rcu_barrier()
>
> Joel Fernandes (Google) (9):
> rcu: Make call_rcu() lazy to save power
> rcu: Refactor code a bit in rcu_nocb_do_flush_bypass()
> rcuscale: Add laziness and kfree tests
> percpu-refcount: Use call_rcu_flush() for atomic switch
> rcu/sync: Use call_rcu_flush() instead of call_rcu
> rcu/rcuscale: Use call_rcu_flush() for async reader test
> rcu/rcutorture: Use call_rcu_flush() where needed
> rxrpc: Use call_rcu_flush() instead of call_rcu()
> rcu/debug: Add wake-up debugging for lazy callbacks
>
> Uladzislau Rezki (2):
> scsi/scsi_error: Use call_rcu_flush() instead of call_rcu()
> workqueue: Make queue_rcu_work() use call_rcu_flush()
>
> Vineeth Pillai (1):
> rcu: shrinker for lazy rcu
>
> drivers/scsi/scsi_error.c | 2 +-
> include/linux/rcupdate.h | 7 ++
> kernel/rcu/Kconfig | 15 +++
> kernel/rcu/lazy-debug.h | 154 +++++++++++++++++++++++++++
> kernel/rcu/rcu.h | 8 ++
> kernel/rcu/rcuscale.c | 70 +++++++++++-
> kernel/rcu/rcutorture.c | 16 +--
> kernel/rcu/sync.c | 2 +-
> kernel/rcu/tiny.c | 2 +-
> kernel/rcu/tree.c | 149 ++++++++++++++++++--------
> kernel/rcu/tree.h | 12 ++-
> kernel/rcu/tree_exp.h | 2 +-
> kernel/rcu/tree_nocb.h | 217 ++++++++++++++++++++++++++++++++------
> kernel/workqueue.c | 2 +-
> lib/percpu-refcount.c | 3 +-
> net/rxrpc/conn_object.c | 2 +-
> 16 files changed, 565 insertions(+), 98 deletions(-)
> create mode 100644 kernel/rcu/lazy-debug.h
>
> --
> 2.38.0.413.g74048e4d9e-goog
>