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

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Date:   Tue, 8 Dec 2020 13:51:04 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 00/19] rcu/nocb: De-offload and re-offload support v4

Hi Boqun Feng,

On Tue, Dec 08, 2020 at 10:41:31AM +0800, Boqun Feng wrote:
> 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?

Thanks a lot for reviewing and testing. You seem to have tested with the right
options, I have nothing better to suggest. Plus I'm glad you tested on
ARM64. x86 is the only target I have tested so far.