Messages in this thread | | | Date | Mon, 28 Sep 2020 14:50:37 +0200 | From | Greg KH <> | Subject | Re: [PATCH 2/2] printk: Make the console flush configurable in hotplug path |
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On Sun, Sep 27, 2020 at 07:05:34PM -0700, psodagud@codeaurora.org wrote: > On 2020-09-24 11:21, Thomas Gleixner wrote: > > On Thu, Sep 24 2020 at 08:33, Greg KH wrote: > > > On Wed, Sep 23, 2020 at 05:08:32PM -0700, Prasad Sodagudi wrote: > > > > +config CONSOLE_FLUSH_ON_HOTPLUG > > > > + bool "Enable console flush configurable in hot plug code path" > > > > + depends on HOTPLUG_CPU > > > > + def_bool n > > > > > > n is the default, no need to list it. > > > > > > > + help > > > > + In cpu hot plug path console lock acquire and release causes the > > > > + console to flush. If console lock is not free hot plug latency > > > > + increases. So make console flush configurable in hot plug path > > > > + and default disabled to help in cpu hot plug latencies. > > > > > > Why would you not want this option? > > > > > > Why isn't this just a bugfix? > > > > Because it's the normal behaviour of console lock and there are > > gazillion other ways to delay stuff in the hotplug path. > > > > CPU hotplug is not meant to be a high speed operation and if people > > think they need it to be fast then its pretty much guaranteed that they > > want it for the completely wrong reasons. > > > > This #ifdef tinkering is just digusting especially as it just tackles an > > obvious way how to delay timer migration, but does not address the > > underlying root cause. > > > > Hi tglx, > > Yes. I agree with you that there are other conditions, which could delay the > hotplug operation. But this console > flushing is not needed in the hotplug path. In the hotplug path, a core is > trying printing messages > from other core(by design of printk), delays the whole hotplug operation and > timers migration. As timers > migration gets delayed, it would impact the systems stability in device > stability testing. > To avoid timers delay in the timer migration in debug builds has to choose > this option. > > I thought of changing the timers and irq migration as priority callbacks in > the hotplug out operation > but I observed some comments like shown below. I was under impression that, > it is hard to find all this > type of conditions, so started tinkering hotplug path by changing the log > levels. > These changes helped on Qualcomm platforms testing. > /* > * On the tear-down path, timers_dead_cpu() must be invoked > * before blk_mq_queue_reinit_notify() from notify_dead(), > * otherwise a RCU stall occurs. > */ > [CPUHP_TIMERS_PREPARE] = { > .name = "timers:prepare", > .startup.single = timers_prepare_cpu, > .teardown.single = timers_dead_cpu, > }, > > Another reason for adding #ifdef is that, I was not clear why console flush > is need cpuhp callback and thought > there might be some use cases and console flush use case might not be valid > for all the users of cpu hotplug. > I will try to explore the changing the callback order to complete the timers > and irq migration early in the hotplug operation. > > Let me put some use cases of hotplug and why hotplug and hotplug latency is > important from testing point of view. > 1) Secondary cpus are hotplug out during the device suspend and hotplug in > during the resume. So cpu hotplug operation is important production devices > point of view as user presses the power key many times.
But what does suspend/resume have to do with this? Why not do just an offline operation instead of unplugging the whole cpu?
> 2) sysfs nodes (/sys/devices/ststem/cpu/cpu4/oneline) are present from linux > kernel, so test team wants to test cpu hotplug. There could be issues with > in generic kernel, device drivers or firmware(psci calls handling from > firmware). There could be issues with device drivers or firmware and test > teams can not leave the hotplug untested in builds.
Your change isn't for testing things, speed doesn't matter when writing to sysfs nodes, right?
> 3) Linux kernel also gave provision to register call backs with cpu hotplug > framework(CPUHP_AP_ONLINE_DYN) dynamic callbacks. > 3002 ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, > "printk:online", > 3003 console_cpu_notify, NULL); > So test team wants to test if any in tree or out of tree modules have any > issues with registered call backs or not.
Again, how is this a speed issue?
> 4) Tracing of the cpuhp operation is important to find whether upstream > changes or out of tree modules(or firmware changes) caused latency > regression or not.
But cpu hotplug is not deterministic, so how does latency matter here?
confused as to the real problem here...
greg k-h
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