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]

Message-ID: <Y/z0fHHYdxEXcWMT@pc636>
Date:   Mon, 27 Feb 2023 19:20:44 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Uladzislau Rezki <urezki@...il.com>, paulmck@...nel.org,
        "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 01:15:47PM -0500, Joel Fernandes wrote:
> 
> 
> > On Feb 27, 2023, at 1:06 PM, Uladzislau Rezki <urezki@...il.com> wrote:
> > 
> > On Mon, Feb 27, 2023 at 10:16:51AM -0500, Joel Fernandes wrote:
> >>> 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?
> >> 
> > Why "rcu_boot_ended" is not enough? As i see right after that an "init"
> > process or shell or panic is going to be invoked by the kernel. It basically
> > indicates that a kernel is fully functional.
> > 
> > Or an idea to wait even further? Until all kernel modules are loaded by
> > user space.
> 
> I mentioned in commit message it is daemons, userspace initialization etc. There is a lot of userspace booting up as well and using the kernel while doing so.
> 
> So, It does not make sense to me to mark kernel as booted too early. And no harm in adding some builtin kernel hysteresis. What am I missing?
> 
Than it is up to user space to decide when it is ready in terms of "boot completed".

--
Uladzislau Rezki