Difference between logical and physical cpu hotplug

[Srivatsa Bhat]

On Thu, Aug 18, 2011 at 11:40 PM, Srivatsa Bhat <bhat.srivatsa at gmail.com>wrote:

>
>
> On Thu, Aug 18, 2011 at 10:44 PM, Vaibhav Jain <vjoss197 at gmail.com> wrote:
>
>>
>>
>> On Thu, Aug 18, 2011 at 9:02 AM, srivatsa bhat <bhat.srivatsa at gmail.com>wrote:
>>
>>> Hi Vaibhav,
>>>
>>> On Thu, Aug 18, 2011 at 8:24 PM, Vaibhav Jain <vjoss197 at gmail.com>wrote:
>>>
>>>> Hi,
>>>>
>>>> I talked to a friend of mine and he suggested that
>>>> in a logical offline state the cpu is powered on and ready to execute
>>>> instructions
>>>> just that the kernel is not aware of it. But in case of physical offline
>>>> state the cpu
>>>> is powered off and cannot run.
>>>> Are you saying something similar ?
>>>>
>>>> Yes, you are right, mostly.
>>> When you try to logically offline a CPU, the kernel will do task
>>> migration (i.e., move out all the tasks running on that CPU to other CPUs in
>>> the system) and it ensures that it doesn't need that CPU anymore. This also
>>> means that, from now on, the context of that CPU need not be saved (because
>>> the kernel has moved that CPU's tasks elsewhere). At this point, it is as if
>>> the kernel is purposefully using only a subset of the available CPUs. This
>>> step is a necessary prerequisite to do physical CPU offline later on.
>>>
>>> But I don't think CPU power ON or OFF is the differentiating factor
>>> between logical and physical offlining. In logical offline, you still have
>>> the CPUs in the system but you just tell the kernel not to use them. At this
>>> stage, you can power off your CPU, to save power for example.
>>> But in physical offline, from a software perspective, you do additional
>>> work at the firmware level (apart from logical offlining at the OS level),
>>> to ensure that physically plugging out the CPUs will not affect the running
>>> system in any way.
>>>
>>> Please note that you can logically online and offline the same CPUs over
>>> and over again without rebooting the system. Here, while onlining a CPU
>>> which was offlined previously, the kernel follows almost the same sequence
>>> which it normally follows while booting the CPUs during full system booting.
>>>
>>> Also one more thing to be noted is that, to be able to physically
>>> hot-plug CPUs, apart from OS and firmware support, you also need the
>>> hardware to support this feature. That is, the electrical wiring to the
>>> individual CPUs should be such that plugging them in and out does not
>>> interfere with the functioning of the rest of the system. As of today, there
>>> are only a few systems that support physical CPU-hotplug. But you can do
>>> logical CPU hotplug easily, by configuring the kernel appropriately during
>>> compilation, as you have noted in one of your previous mails.
>>>
>>> Regards,
>>> Srivatsa S. Bhat
>>>
>>
>>
>> Hi Srivatsa,
>>
>> That was great explanation! Thanks!
>> I have just one more query. You mentioned above that " the kernel follows
>> almost the same *sequence *which it normally follows while booting the
>> CPUs during full system booting."
>>
>> Can you please explain this sequence a little ?
>>
>>
> Hi Vaibhav,
>
> I'll try to outline a very high level view of what happens while booting an
> SMP (Symmetric Multi-Processor) system. Instead of going through the entire
> boot sequence, let me just highlight only the part that is of interest in
> this discussion: booting multiple CPUs.
>
> The "boot processor" is the one which is booted first while booting a
> system. On x86 architecture, CPU 0 is always the boot processor. Hence, if
> you have observed, you cannot offline CPU0 using CPU hot-plugging on an x86
> machine. (On an Intel box, the file /sys/devices/system/cpu/cpu0/online is
> purposefully absent, for this reason!). But in other architectures, this
> might not be the case. For example on POWER architecture, any processor in
> the system can act as the boot processor.
>
> Once the boot processor does its initialization, the other processors,
> known as "secondary processors or application processors (APs)" are
> booted/initialized. Here, obviously some synchronization mechanism is
> necessary to ensure that this order is followed. So in Linux, we use 2
> bitmasks called "cpu_callout_mask" and "cpu_callin_mask". These bitmasks are
> used to indicate the processors available in the system.
>
> Once the boot processor initializes itself, it updates cpu_callout_mask to
> indicate which secondary processor (or application processor AP) can
> initialize itself next (for example, the boot processor sets a particular
> bit as 1 in the cpu_callout_mask). On the other hand, the secondary
> processor would have done some very basic initialization till then and will
> be testing the value of 'cpu_callout_mask' in a while loop to see if its
> number has been "called out" by the boot processor. Only after the boot
> processor "calls out" this AP, this AP will continue the rest of its
> initialization and completes it.
>
> Once the AP completes its initialization, it reports back to the boot
> processor by setting its number in the cpu_callin_mask. As expected, the
> boot processor would have been waiting in a while loop on cpu_callin_mask to
> see if this AP booted OK or not. Once it finds that the cpu_callin_mask for
> this AP has been set, the boot processor follows the same procedure to boot
> other APs: i.e., it updates cpu_callout_mask and waits for the corresponding
> entry to be set in cpu_callin_mask by that AP and so on. This process
> continues until all the APs are booted up.
>
> Of course, each of these "waiting" times (of both boot processor and APs)
> are capped by some preset value, say for example 5 seconds. If some AP takes
> more than that time to boot, the boot processor declares that the AP could
> not boot and takes appropriate action (like clearing its bit in
> cpu_callout_mask and logically removing that AP from its tables etc,
> effectively forgetting about that processor). Similarly while the APs wait
> for the boot processor to call them out, if the boot processor does not call
> them within a given time period, they declare kernel panic.
>
> Here are some references, if you are interested in more details:
>
> Linux kernel source code:
> 1. linux/arch/x86/kernel/smpboot.c : start_secondary() and smp_callin()
>     These are the functions executed by the APs (secondary or application
> processors). Actually smp_callin() is called within start_secondary() which
> is the primary function executed by APs.
>
> 2. linux/arch/x86/kernel/smpboot.c :  do_boot_cpu()
>
       This is executed by the boot processor.  You can look up other
important functions such as native_cpu_up().

    General SMP booting info:
    1. http://www.cheesecake.org/sac/smp.html

[ Sorry, I accidentally sent the earlier mail before composing the text
fully. ]

Regards,
Srivatsa S. Bhat
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