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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Mon, 27 Feb 2023 10:16:51 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     paulmck@...nel.org
Cc:     "Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>, linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.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 9:55 AM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Mon, Feb 27, 2023 at 08:22:06AM -0500, Joel Fernandes wrote:
> >
> >
> > > On Feb 27, 2023, at 2:53 AM, Zhuo, Qiuxu <qiuxu.zhuo@...el.com> wrote:
> > >
> > > 
> > >>
> > >> From: Joel Fernandes (Google) <joel@...lfernandes.org>
> > >> Sent: Saturday, February 25, 2023 11:34 AM
> > >> To: linux-kernel@...r.kernel.org
> > >> Cc: Joel Fernandes (Google) <joel@...lfernandes.org>; Frederic Weisbecker
> > >> <frederic@...nel.org>; Lai Jiangshan <jiangshanlai@...il.com>; linux-
> > >> doc@...r.kernel.org; Paul E. McKenney <paulmck@...nel.org>;
> > >> rcu@...r.kernel.org
> > >> Subject: [PATCH RFC v2] rcu: Add a minimum time for marking boot as
> > >> completed
> > >>
> > >> On many systems, a great deal of boot happens after the kernel thinks the
> > >> boot has completed. It is difficult to determine if the system has really
> > >> booted from the kernel side. Some features like lazy-RCU can risk slowing
> > >> down boot time if, say, a callback has been added that the boot
> > >> synchronously depends on.
> > >>
> > >> Further, it is better to boot systems which pass 'rcu_normal_after_boot' to
> > >> stay expedited for as long as the system is still booting.
> > >>
> > >> For these reasons, this commit adds a config option
> > >> 'CONFIG_RCU_BOOT_END_DELAY' and a boot parameter
> > >> rcupdate.boot_end_delay.
> > >>
> > >> By default, this value is 20s. A system designer can choose to specify a value
> > >> here to keep RCU from marking boot completion.  The boot sequence will not
> > >> be marked ended until at least boot_end_delay milliseconds have passed.
> > >
> > > Hi Joel,
> > >
> > > Just some thoughts on the default value of 20s, correct me if I'm wrong :-).
> > >
> > > Does the OS with CONFIG_PREEMPT_RT=y kernel concern more about the
> > > real-time latency than the overall OS boot time?
> >
> > But every system has to boot, even an RT system.
> >
> > >
> > > If so, we might make rcupdate.boot_end_delay = 0 as the default value
> > > (NOT the default 20s) for CONFIG_PREEMPT_RT=y kernels?
> >
> > Could you measure how much time your RT system takes to boot before the application runs?
> >
> > I can change it to default 0 essentially NOOPing it, but I would rather have a saner default (10 seconds even), than having someone forget to tune this for their system.
>
> Provide a /sys location that the userspace code writes to when it
> is ready?  Different systems with different hardware and software
> configurations are going to take different amounts of time to boot,
> correct?

I could add a sysfs node, but I still wanted this patch as well
because I am wary of systems where yet more userspace changes are
required. I feel the kernel should itself be able to do this. Yes, it
is possible the system completes "booting" at a different time than
what the kernel thinks. But it does that anyway (even without this
patch), so I am not seeing a good reason to not do this in the kernel.
It is also only a minimum cap, so if the in-kernel boot takes too
long, then the patch will have no effect.

Thoughts?

thanks,

 - Joel