linux-kernel - Re: [PATCH 00/19] rcu/context-tracking: Merge RCU eqs-dynticks counter to context tracking

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Message-ID: <c6333d9ebf5fcf03c1cf62071336c110a013a762.camel@kernel.org>
Date:   Fri, 11 Mar 2022 12:37:55 +0100
From:   nicolas saenz julienne <nsaenz@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     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 00/19] rcu/context-tracking: Merge RCU eqs-dynticks
 counter to context tracking

On Wed, 2022-03-02 at 16:47 +0100, 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
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> 	rcu/context-tracking
> 
> HEAD: e4eaff86ec91c1cbde9a113cf5232dac9f897337
> 
> Thanks,
> 	Frederic
> ---

I've been testing patches 1 to 18 on my nohz_full setup. Everything is looking
good so far.

Regards,
Nicolas