linux-kernel - Re: [PATCH RFC v2] rcu: Add a minimum time for marking boot as completed

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Message-ID: <Y/0/dnmIk508sidK@lothringen>
Date:   Tue, 28 Feb 2023 00:40:38 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Uladzislau Rezki <urezki@...il.com>,
        "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>, linux-kernel@...r.kernel.org,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org, rcu@...r.kernel.org
Subject: Re: [PATCH RFC v2] rcu: Add a minimum time for marking boot as
 completed

On Mon, Feb 27, 2023 at 03:05:02PM -0800, Paul E. McKenney wrote:
> On Mon, Feb 27, 2023 at 02:10:30PM -0500, Joel Fernandes wrote:
> 
> The combination of sysfs manipulated by userspace and a kernel failsafe
> makes sense to me.  Especially if by default triggering the failsafe
> splats.  That way, bugs where userspace fails to update the sysfs file
> get caught.
> 
> The non-default silent-failsafe mode is also useful to allow some power
> savings in advance of userspace getting the sysfs updating in place.
> And of course the default splatting setup can be used in internal testing
> with the release software being more tolerant of userspace foibles.

I'm wondering, this is all about CONFIG_RCU_LAZY, right? Or does also expedited
GP turned off a bit early or late on boot matter for anybody in practice?

So shouldn't we disable lazy callbacks by default when CONFIG_RCU_LAZY=y and then
turn it on with "sysctl kernel.rcu.lazy=1" only whenever userspace feels ready
about it? We can still keep the current call to rcu_end_inkernel_boot().

And if suddenly disabling lazy by default is an ABI breakage we can still add
CONFIG_RCU_LAZY_DEFAULT_DISABLED.