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

[Uladzislau Rezki]

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@xxxxxxxxx>
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> 
> 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.

--
Uladzislau Rezki