Hi Elliot + Sudeep On 10/27/20 10:06, Elliot Berman wrote: > > On 10/26/2020 10:12 AM, Peter Zijlstra wrote: > > On Mon, Oct 26, 2020 at 10:08:47AM -0700, psodagud@xxxxxxxxxxxxxx wrote: > > > On 2020-10-23 14:59, Thomas Gleixner wrote: > > > > On Thu, Oct 22 2020 at 15:04, Elliot Berman wrote: > > > > > In a heterogeneous multiprocessor system, specifying the 'maxcpus' > > > > > parameter on kernel command line does not provide sufficient control > > > > > over which CPUs are brought online at kernel boot time, since CPUs may > > > > > have nonuniform performance characteristics. Thus, add bootcpus kernel > > > > > parameter to control which CPUs should be brought online during kernel > > > > > boot. When both maxcpus and bootcpus is set, the more restrictive of > > > > > the > > > > > two are booted. > > > > > > > > What for? 'maxcpus' is a debug hack at best and outright dangerous on > > > > certain architectures. Why do we need more of that? Just let the machine > > > > boot and offline the CPUs from user space. > > > > > > Hi Thomas and Peter, > > > > > > Based on my understanding with maxcpus option provides, maximum no of CPUs > > > are brough up during the device boot up. There is a different case, in which > > > we want to restrict which CPUs to be brough up. > > > On a system with 8 cpus, if we set maxcpus as 3, cpu0, cpu1, and cpu2 are > > > brough up during the bootup. For example, if we want to bring core0, core3 > > > and core4 current maxcpu(as 3) setting would not help us. > > > On some platform we want the flexibility on which CPUs to bring up during > > > the device bootup. bootcpus command line is helping to bring specific CPUs > > > and these patches are working downstream. > > > > That's a lot of words, but exactly 0 on _WHY_ you would want to do that. > > > > We find the ability to limit the number of cpus brought online at bootup > useful, and to possibly later enable those cores. One use case is when > device is undergoing initial testing is to use bootcpus to limit bootup to > only a couple cores and later bring up the other cores for a controlled > stress test. A core brought up during boot is also running device > initialization. Besides being useful for SoC vendor bringup which typically > occurs downstream, this particular use case could be exercised by developer > of upstream support for a SoC when initial CPU settings are being > determined. > > Another use case is if user wishes to limit bootup only to the smaller or > bigger cores. maxcpus= is not sufficient here to ensure that only those > cores are booted since it limits only to the first N cores, which may not be > the desired small or big cores. User may want to bring up only the smaller > cores during bootup for thermal reasons. For instance, device may be later > sufficiently charged such that boot up of the bigger cores is now > permissible. Relying on thermal drivers to later take care of putting core > into lower power idle may not occur until much later in boot (for instance, > if the governor is a module). I would have thought that FW/SCP would have the power to block booting up the CPUs if it deemed that to be unsafe. Any thoughts Sudeep? Thanks -- Qais Yousef
