linux-kernel - Re: [PATCH v3 resend 0/6] Implement call_rcu

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Date:   Wed, 10 Aug 2022 22:23:23 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, frederic@...nel.org, paulmck@...nel.org,
        rostedt@...dmis.org, rcu@...r.kernel.org
Subject: Re: [PATCH v3 resend 0/6] Implement call_rcu_lazy() and miscellaneous
 fixes



On 8/8/2022 11:45 PM, Joel Fernandes (Google) wrote:
> Just a refresh of v3 with one additional debug patch. v3's cover letter is here:
>  https://lore.kernel.org/all/20220713213237.1596225-1-joel@joelfernandes.org/
> 
> I just started working on this again while I have some time during paternity
> leave ;-) So I thought I'll just send it out again. No other changes other
> than that 1 debug patch I added on the top.
> 
> Next I am going to go refine the power results as mentioned in Paul's comments
> on the last cover letter.

Side note: Here is another big selling point for call_rcu_lazy().
Instead of _lazy(), if you just increased jiffies_till_first_fqs, and
slowed *all* call_rcu() down to achieve the same effect, that would
affect percpu refcounters switching to atomic-mode, for example.

They switch to atomic mode by calling __percpu_ref_switch_mode() which
is called by percpu_ref_switch_to_atomic_sync().

This will slow this call down for the full lazy duration which will slow
down suspend in blk_pre_runtime_suspend().

This is why, we cannot assume call_rcu() users will mostly just want to
free memory. There could be cases just like this, and just blanket slow
down of call_rcu() might bite at unexpected times.

I am going to add this as a selling point for selective lazyfication
(hey I get to invent words while I'm inventing new features), to my
cover letter and slides.

 - Joel



> 
> Joel Fernandes (Google) (5):
> rcu: Introduce call_rcu_lazy() API implementation
> rcuscale: Add laziness and kfree tests
> fs: Move call_rcu() to call_rcu_lazy() in some paths
> rcutorture: Add test code for call_rcu_lazy()
> debug: Toggle lazy at runtime and change flush jiffies
> 
> Vineeth Pillai (1):
> rcu: shrinker for lazy rcu
> 
> fs/dcache.c                                   |   4 +-
> fs/eventpoll.c                                |   2 +-
> fs/file_table.c                               |   2 +-
> fs/inode.c                                    |   2 +-
> include/linux/rcu_segcblist.h                 |   1 +
> include/linux/rcupdate.h                      |   6 +
> include/linux/sched/sysctl.h                  |   3 +
> kernel/rcu/Kconfig                            |   8 +
> kernel/rcu/rcu.h                              |  12 +
> kernel/rcu/rcu_segcblist.c                    |  15 +-
> kernel/rcu/rcu_segcblist.h                    |  20 +-
> kernel/rcu/rcuscale.c                         |  74 +++++-
> kernel/rcu/rcutorture.c                       |  60 ++++-
> kernel/rcu/tree.c                             | 131 ++++++----
> kernel/rcu/tree.h                             |  10 +-
> kernel/rcu/tree_nocb.h                        | 246 +++++++++++++++---
> kernel/sysctl.c                               |  17 ++
> .../selftests/rcutorture/configs/rcu/CFLIST   |   1 +
> .../selftests/rcutorture/configs/rcu/TREE11   |  18 ++
> .../rcutorture/configs/rcu/TREE11.boot        |   8 +
> 20 files changed, 536 insertions(+), 104 deletions(-)
> create mode 100644 tools/testing/selftests/rcutorture/configs/rcu/TREE11
> create mode 100644 tools/testing/selftests/rcutorture/configs/rcu/TREE11.boot
> 
> --
> 2.37.1.559.g78731f0fdb-goog
>