Re: [PATCH v3] rcu: Add a minimum time for marking boot as completed

Joel Fernandes
2023-03-15T12:19:29Z

On Wed, Mar 08, 2023 at 11:14:40AM +0100, Uladzislau Rezki wrote:
> On Tue, Mar 07, 2023 at 08:48:52AM -0500, Joel Fernandes wrote:
> > On Tue, Mar 7, 2023 at 8:40 AM Uladzislau Rezki <urezki@gmail.com> wrote:
> > >
> > > On Tue, Mar 07, 2023 at 02:01:54PM +0100, Frederic Weisbecker wrote:
> > > > On Fri, Mar 03, 2023 at 09:38:51PM +0000, Joel Fernandes (Google) wrote:
> > > > > On many systems, a great deal of boot (in userspace) 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 expedited callbacks
> > > > > can get unexpedited way earlier than it should be, thus slowing down
> > > > > boot (as shown in the data below).
> > > > >
> > > > > For these reasons, this commit adds a config option
> > > > > 'CONFIG_RCU_BOOT_END_DELAY' and a boot parameter rcupdate.boot_end_delay.
> > > > > Userspace can also make RCU's view of the system as booted, by writing the
> > > > > time in milliseconds to: /sys/module/rcupdate/parameters/rcu_boot_end_delay
> > > > > Or even just writing a value of 0 to this sysfs node.
> > > > > However, under no circumstance will the boot be allowed to end earlier
> > > > > than just before init is launched.
> > > > >
> > > > > The default value of CONFIG_RCU_BOOT_END_DELAY is chosen as 15s. This
> > > > > suites ChromeOS and also a PREEMPT_RT system below very well, which need
> > > > > no config or parameter changes, and just a simple application of this patch. A
> > > > > system designer can also choose a specific value here to keep RCU from marking
> > > > > boot completion. As noted earlier, RCU's perspective of the system as booted
> > > > > will not be marker until at least rcu_boot_end_delay milliseconds have passed
> > > > > or an update is made via writing a small value (or 0) in milliseconds to:
> > > > > /sys/module/rcupdate/parameters/rcu_boot_end_delay.
> > > > >
> > > > > One side-effect of this patch is, there is a risk that a real-time workload
> > > > > launched just after the kernel boots will suffer interruptions due to expedited
> > > > > RCU, which previous ended just before init was launched. However, to mitigate
> > > > > such an issue (however unlikely), the user should either tune
> > > > > CONFIG_RCU_BOOT_END_DELAY to a smaller value than 15 seconds or write a value
> > > > > of 0 to /sys/module/rcupdate/parameters/rcu_boot_end_delay, once userspace
> > > > > boots, and before launching the real-time workload.
> > > > >
> > > > > Qiuxu also noted impressive boot-time improvements with earlier version
> > > > > of patch. An excerpt from the data he shared:
> > > > >
> > > > > 1) Testing environment:
> > > > > OS : CentOS Stream 8 (non-RT OS)
> > > > > Kernel : v6.2
> > > > > Machine : Intel Cascade Lake server (2 sockets, each with 44 logical threads)
> > > > > Qemu args : -cpu host -enable-kvm, -smp 88,threads=2,sockets=2, …
> > > > >
> > > > > 2) OS boot time definition:
> > > > > The time from the start of the kernel boot to the shell command line
> > > > > prompt is shown from the console. [ Different people may have
> > > > > different OS boot time definitions. ]
> > > > >
> > > > > 3) Measurement method (very rough method):
> > > > > A timer in the kernel periodically prints the boot time every 100ms.
> > > > > As soon as the shell command line prompt is shown from the console,
> > > > > we record the boot time printed by the timer, then the printed boot
> > > > > time is the OS boot time.
> > > > >
> > > > > 4) Measured OS boot time (in seconds)
> > > > > a) Measured 10 times w/o this patch:
> > > > > 8.7s, 8.4s, 8.6s, 8.2s, 9.0s, 8.7s, 8.8s, 9.3s, 8.8s, 8.3s
> > > > > The average OS boot time was: ~8.7s
> > > > >
> > > > > b) Measure 10 times w/ this patch:
> > > > > 8.5s, 8.2s, 7.6s, 8.2s, 8.7s, 8.2s, 7.8s, 8.2s, 9.3s, 8.4s
> > > > > The average OS boot time was: ~8.3s.
> > > > >
> > > > > Tested-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
> > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > >
> > > > I still don't really like that:
> > > >
> > > > 1) It feels like we are curing a symptom for which we don't know the cause.
> > > > Which RCU write side caller is the source of this slow boot? Some tracepoints
> > > > reporting the wait duration within synchronize_rcu() calls between the end of
> > > > the kernel boot and the end of userspace boot may be helpful.
> > > >
> > > > 2) The kernel boot was already covered before this patch so this is about
> > > > userspace code calling into the kernel. Is that piece of code also called
> > > > after the boot? In that case are we missing a conversion from
> > > > synchronize_rcu() to synchronize_rcu_expedited() somewhere? Because then
> > > > the problem is more general than just boot.
> > > >
> > > > This needs to be analyzed first and if it happens that the issue really
> > > > needs to be fixed with telling the kernel that userspace has completed
> > > > booting, eg: because the problem is not in a few callsites that need conversion
> > > > to expedited but instead in the accumulation of lots of calls that should stay
> > > > as is:
> > > >
> > > > 3) This arbitrary timeout looks dangerous to me as latency sensitive code
> > > > may run right after the boot. Either you choose a value that is too low
> > > > and you miss the optimization or the value is too high and you may break
> > > > things.
> > > >
> > > > 4) This should be fixed the way you did:
> > > > a) a kernel parameter like you did
> > > > b) The init process (systemd?) tells the kernel when it judges that userspace
> > > > has completed booting.
> > > > c) Make these interfaces more generic, maybe that information will be useful
> > > > outside RCU. For example the kernel parameter should be
> > > > "user_booted_reported" and the sysfs (should be sysctl?):
> > > > kernel.user_booted = 1
> > > > d) But yuck, this means we must know if the init process supports that...
> > > >
> > > > For these reasons, let's make sure we know exactly what is going on first.
> > > >
> > > > Thanks.
> > > Just add some notes and thoughts. There is a rcupdate.rcu_expedited=1
> > > parameter that can be used during the boot. For example on our devices
> > > to speedup a boot we boot the kernel with rcu_expedited:
> > >
> > > XQ-DQ54:/ # cat /proc/cmdline
> > > stack_depot_disable=on kasan.stacktrace=off kvm-arm.mode=protected cgroup_disable=pressure console=ttyMSM0,115200n8 loglevel=6 kpti=0 log_buf_len=256K kernel.panic_on_rcu_stall=1 service_locator.enable=1 msm_rtb.filter=0x237 rcupdate.rcu_expedited=1 rcu_nocbs=0-7 ftrace_dump_on_oops swiotlb=noforce loop.max_part=7 fw_devlink.strict=1 allow_mismatched_32bit_el0 cpufreq.default_governor=performance printk.console_no_auto_verbose=1 kasan=off sysctl.kernel.sched_pelt_multiplier=4 can.stats_timer=0 pcie_ports=compat irqaffinity=0-2 disable_dma32=on no-steal-acc cgroup.memory=nokmem,nosocket video=vfb:640x400,bpp=32,memsize=3072000 page_owner=on stack_depot_disable=off printk.console_no_auto_verbose=0 nosoftlockup bootconfig buildvariant=userdebug msm_drm.dsi_display0=somc,1_panel: rootwait ro init=/init qcom_geni_serial.con_enabled=0 oembootloader.startup=0x00000001 oembootloader.warmboot=0x00000000 oembootloader.securityflags=0x00000001
> > > XQ-DQ54:/ #
> > >
> > > then a user space can decides if it is needed or not:
> > >
> > > <snip>
> > > rcu_expedited rcu_normal
> > > XQ-DQ54:/ # ls -al /sys/kernel/rcu_*
> > > -rw-r--r-- 1 root root 4096 2023-02-16 09:27 /sys/kernel/rcu_expedited
> > > -rw-r--r-- 1 root root 4096 2023-02-16 09:27 /sys/kernel/rcu_normal
> > > XQ-DQ54:/ #
> > > <snip>
> > >
> > > for lazy we can add "rcu_cb_lazy" parameter and boot the kernel with
> > > true or false. So we can follow and be aligned with rcu_expedited and
> > > rcu_normal parameters.
> >
> > Speaking of aligning, there is also the automated
> > rcu_normal_after_boot boot option correct? I prefer the automated
> > option of doing this. So the approach here is not really unprecedented
> > and is much more robust than relying on userspace too much (I am ok
> > with adding your suggestion *on top* of the automated toggle, but I
> > probably would not have ChromeOS use it if the automated way exists).
> > Or did I miss something?
> >
> According to name of the rcu_end_inkernel_boot() function and a place
> when it is invoked we can conclude that it marks the end of kernel boot
> and it happens before running an "init" process.
>
> With your patch we change a behavior. The initialization occurs not right
> after a kernel is up and running but rather after 15 seconds timeout what
> at least does not correspond to a function name. Apart from that an expected
> behavior might be different. For example some test-suites or smoke tests, etc.
>
> Another thought about "automated boot complete" is we do not know from
> kernel space when it really completes for user space, because from kernel
> space we are done and we can detect it. In this cases a user space is a
> right candidate to say when it is ready.
>
> For example for Android a boot complete happens when a home-screen appears.
> For Chrome OS i think there is something similar. There must be a boot complete
> event in its init scripts or something similar.
>
> This is just my thoughts. I do not really mind but i also do not see a high
> need in having it.

Thanks for your thoughts, perhaps if I am the only one who wants it, then it
is a bad idea. Here's some hoping to get some more time this week to dig
deeper into this... this week has been crazy on the personal front.

thanks,

- Joel