Messages in this thread | | | From | Noam Camus <> | Subject | RE: Reduce Linux boot time on Large scale system | Date | Wed, 19 Apr 2017 10:08:45 +0000 |
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>From: Thomas Gleixner [mailto:tglx@linutronix.de] >Sent: Wednesday, April 19, 2017 11:58 AM >On Wed, 19 Apr 2017, Peter Zijlstra wrote: >> On Tue, Apr 04, 2017 at 04:39:06PM +0000, Noam Camus wrote: >> > Hi Peter & Vineet >> > >> > I wish to reduce boot time of my platform ARC/plat-eznps (4K CPUs). >> > My analysis is that most boot time is spent over cpu_up() for all >> > CPUs Measurements are about 66mS per CPU and Totally over 4 minutes (I got 800MHz cores). >> > >> > I see that smp_init() just iterate over all present cpus one by one. >> > I wish to know if there was an attempt to optimize this with some parallel work? >> > >> > Are you aware of some method / trick that will help me to reduce boot time? >> > Any suggestion how this can be done? >> >> So attempts have been made in the past but Thomas shot them down for >> being gross hacks (they were). >> >> But Thomas has now (mostly) completed rewriting the CPU hotplug >> machinery and he has at some point outlined means of achieving what >> you're after. >> >> I've added him to Cc so he can correct me where I'm wrong, as I've not >> looked into this in much detail after he mucked up all I knew about >> CPU hotplug. >> >> Since each CPU is now responsible for its own bootstrap, we can now >> kick all the CPUs awake without waiting for them to complete the >> online stage. >> >> There might however be code that assumes CPUs come up one at a time, >> so you'll need to audit for that. Its not going to be a trivial thing.
>There are a couple of things to consider.
>First of all we should make the whole 'kick CPU into life' and surrounding magic generic. Every arch has it's own handshake mechanism.
>That would look like this:
>Step BP AP >0-9 [preparatory steps]
>10 [kick cpu into life (arch callback)] >11 [Do initial arch bringup then call in into a generic function ] >12 [handshake (generic)] [handshake (generic)] >13 [more arch specific magic] [more arch specific magic]
>14-20 [ CPU starting ] [ CPU goes online ]
>40 [ CPU active, hotplug done ]
>So the first step in parallelizing this would be:
> for_each_present_cpu(cpu) > cpu_up(target_state = 10);
>i.e. make the allocations and whatever preparatory work needs to be done and kick the CPU into life. The target CPU would intialize the low level stuff and then call into a generic function, which does the generic initialization and then waits for the handshake.
>So the next thing would be:
> for_each_present_cpu(cpu) > cpu_up(target_state = 40);
>This last step has to be single threaded for now because almost all CPU hotplug using facilities rely on the current serialization. There are also code pathes which use get_online_cpus() or cpu_hotplug_disable() to prevent interaction with cpu hotplug.
>The hotplug machinery is already designed so that after the handshake (#12/13] a plugged CPU can bring up itself completely alone, but due to the serialization expectations all over the place this won't work today.
>To make it work, you have to go through every single instance of CPU hotplug callback users and every single site which prevents hotplug via get_online_cpus() or cpu_hotplug_disable() and audit them for concurrency issues and fix them up.
>There might also be interaction required with the state machine, i.e. stop the state progress on a self plugging CPU between two steps to make serialization work.
What would be a good base to start on all above? Would some formal release like v4.8 TAG good enough , or do I need to base on some other specific HEAD (or TAG)?
Thanks, Noam
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