linux-kernel - Re: [PATCH 00/21] rcu/context-tracking: Merge RCU eqs-dynticks counter to context tracking v2

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Date:   Thu, 19 May 2022 17:07:55 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Phil Auld <pauld@...hat.com>,
        Alex Belits <abelits@...vell.com>,
        Xiongfeng Wang <wangxiongfeng2@...wei.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yu Liao <liaoyu15@...wei.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Uladzislau Rezki <uladzislau.rezki@...y.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 00/21] rcu/context-tracking: Merge RCU eqs-dynticks
 counter to context tracking v2

On Wed, May 18, 2022 at 05:33:02PM +0200, Nicolas Saenz Julienne wrote:
> On Tue, 2022-05-03 at 12:00 +0200, Frederic Weisbecker wrote:
> > This mixes up the RCU dynticks counter and the context tracking state
> > updates into a single atomic instruction. This may serve several
> > purposes:
> > 
> > 1) Improve CPU isolation with deferring some disturbances until sensitive
> >    userspace workload completes and goes to the kernel. This can take
> >    several forms, for example smp_call_function_housekeeping() or
> >    on_each_housekeeping_cpu() to enqueue and execute work on all
> >    housekeeping CPUs. Then an atomic operation on ct->state can defer
> >    the work on nohz_full CPUs until they run in kernel (or IPI them
> >    if they are in kernel mode), see this proposal by Peter:
> >    https://lore.kernel.org/all/20210929151723.162004989@infradead.org/#r
> > 
> > 2) Unearth sysidle (https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/commit/?h=sysidle.2017.05.11a&id=fe5ac724d81a3c7803e60c2232718f212f3f38d4)
> >    This feature allowed to shutdown the tick on the last housekeeping
> >    CPU once the rest of the system is fully idle. We needed some proper
> >    fully ordered context tracking for that.
> > 
> > Inspired by Peterz: https://lore.kernel.org/all/20210929151723.162004989@infradead.org
> > 
> > Passed 100 hours of TREE01.
> > 
> > ---
> 
> For the whole series:
> 
> Reviewed-by: Nicolas Saenz Julienne <nsaenzju@...hat.com>
> Tested-by: Nicolas Saenz Julienne <nsaenzju@...hat.com>

I hesitated to apply these but the series has changed a little, although
the fundamental bits remain.

I'll just wait for a confirmation from you on this v3.

Thanks!