Re: [PATCH 00/19] rcu/nocb: De-offload and re-offload support v4

From: Boqun Feng <boqun.feng@gmail.com>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: "Paul E . McKenney" <paulmck@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Neeraj Upadhyay <neeraju@codeaurora.org>,
	Joel Fernandes <joel@joelfernandes.org>,
	Josh Triplett <josh@joshtriplett.org>,
	rcu@vger.kernel.org
Subject: Re: [PATCH 00/19] rcu/nocb: De-offload and re-offload support v4
Date: Tue, 8 Dec 2020 10:41:31 +0800	[thread overview]
Message-ID: <20201208024131.GI3025@boqun-archlinux> (raw)
In-Reply-To: <20201113121334.166723-1-frederic@kernel.org>

Hi Frederic,

On Fri, Nov 13, 2020 at 01:13:15PM +0100, Frederic Weisbecker wrote:
> This keeps growing up. Rest assured, most of it is debug code and sanity
> checks.
> 
> Boqun Feng found that holding rnp lock while updating the offloaded
> state of an rdp isn't needed, and he was right despite my initial
> reaction. The sites that read the offloaded state while holding the rnp
> lock are actually protected because they read it locally in a non
> preemptible context.
> 
> So I removed the rnp lock in "rcu/nocb: De-offloading CB". And just to
> make sure I'm not missing something, I added sanity checks that ensure
> we always read the offloaded state in a safe way (3 last patches).
> 
> Still passes TREE01 (but I had to fight!)
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> 	rcu/nocb-toggle-v4
> 
> HEAD: 579e15efa48fb6fc4ecf14961804051f385807fe
> 

This whole series look good to me, plus I've run a test, so far
everything seems working ;-) Here is my setup for the test:

I'm using a ARM64 guest (running on Hyper-V) to do the test, and the
guest has 8 VCPUs. The code I'm using is v5.10-rc6 + Hyper-V ARM64 guest
support [1] + your patchset (I actually did a merge from your
rcu/nocb-toggle-v5 branch, because IIUC some modification for rcutorture
is still in Paul's tree). I compiled with my normal configuration for
ARM64 Hyper-V guest plus TREE01, boot the kernel with:

	ignore_loglevel rcutree.gp_preinit_delay=3 rcutree.gp_init_delay=3 rcutree.gp_cleanup_delay=3 rcu_nocbs=0-1,3-7	

and run rcutorture via:

	modprobe rcutorture nocbs_nthreads=8 nocbs_toggle=1000 fwd_progress=0

I ran the rcutorture twice, one last for a week or so and one for a day
or two and I didn't observe any problem so far. The latest test summary
is:

	[...] rcu-torture: rtc: 00000000f794686f ver: 2226396 tfle: 0 rta: 2226397 rtaf: 0 rtf: 2226385 rtmbe: 0 rtmbkf: 0/1390141 rtbe: 0 rtbke: 0 rtbre: 0 rtbf: 0 rtb: 0 nt: 181415346 onoff: 0/0:0/0 -1,0:-1,0 0:0 (HZ=1000) barrier: 0/0:0 read-exits: 108102 nocb-toggles: 306964:306974 

Is there anything I'm missing for a useful test? Do you have other setup
(kernel cmdline or rcutorture parameters) that you want me to try?

Regards,
Boqun

> Thanks,
> 	Frederic
> ---
> 
> Frederic Weisbecker (19):
>       rcu/nocb: Turn enabled/offload states into a common flag
>       rcu/nocb: Provide basic callback offloading state machine bits
>       rcu/nocb: Always init segcblist on CPU up
>       rcu/nocb: De-offloading CB kthread
>       rcu/nocb: Don't deoffload an offline CPU with pending work
>       rcu/nocb: De-offloading GP kthread
>       rcu/nocb: Re-offload support
>       rcu/nocb: Shutdown nocb timer on de-offloading
>       rcu: Flush bypass before setting SEGCBLIST_SOFTIRQ_ONLY
>       rcu/nocb: Set SEGCBLIST_SOFTIRQ_ONLY at the very last stage of de-offloading
>       rcu/nocb: Only cond_resched() from actual offloaded batch processing
>       rcu/nocb: Process batch locally as long as offloading isn't complete
>       rcu/nocb: Locally accelerate callbacks as long as offloading isn't complete
>       tools/rcutorture: Support nocb toggle in TREE01
>       rcutorture: Remove weak nocb declarations
>       rcutorture: Export nocb (de)offloading functions
>       cpu/hotplug: Add lockdep_is_cpus_held()
>       timer: Add timer_curr_running()
>       rcu/nocb: Detect unsafe checks for offloaded rdp
> 
> 
>  include/linux/cpu.h                                |   1 +
>  include/linux/rcu_segcblist.h                      | 119 +++++-
>  include/linux/rcupdate.h                           |   4 +
>  include/linux/timer.h                              |   2 +
>  kernel/cpu.c                                       |   7 +
>  kernel/rcu/rcu_segcblist.c                         |  13 +-
>  kernel/rcu/rcu_segcblist.h                         |  45 ++-
>  kernel/rcu/rcutorture.c                            |   3 -
>  kernel/rcu/tree.c                                  |  49 ++-
>  kernel/rcu/tree.h                                  |   2 +
>  kernel/rcu/tree_plugin.h                           | 416 +++++++++++++++++++--
>  kernel/time/timer.c                                |  13 +
>  .../selftests/rcutorture/configs/rcu/TREE01.boot   |   4 +-
>  13 files changed, 614 insertions(+), 64 deletions(-)