Re: [PATCH] smp: Add bootcpus parameter to boot subset of CPUs
From: Elliot Berman
Date: Thu Oct 29 2020 - 17:37:54 EST
On 10/28/2020 8:15 AM, Sudeep Holla wrote:
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.
Either offline "unwanted" CPUs from user space. If that is not possible
for whatever thermal reasons, we need to check if we can disable them in
the DT like ACPI does. IIUC, it is not supported for some reasons I need
to recall/check, can't remember that now. If that is not possible, make
those nodes disappear in the bootloader ?
If I disappear the cpu nodes in bootloader, then I can't later online
these cpus back up when policy permits. In our experience, there is a
performance hit of ~100ms to modify any devicetree node in bootloader,
which is significant on a commercial device wanting to disable bootup of
certain cores for thermal.
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.
Why not try single core instead of couple of core and add the needed ones
for the user-space ?
In some instances, we have seen that further debugging is needed from
firmware or hardware teams. In these instances, we wanted device to
still be able to do SMP boot, but with a few cores disabled.
In the case where commercial device is using feature for thermal, device
should boot multiple small cores. Booting only one core means we would
not be able to use all possible cores to maximum extent possible in this
thermal case.
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.
I think it is more like *desire* to run with whatever battery life is left
rather than *unsafe* to bring up the core.
Also not sure if we can put such battery life related policies in the
firmware. If there is a thermal constraint, I am sure f/w will and must
refuse to boot the core. I doubt if we are talking about that here. It is
more a policy to extract max out of the battery life left, at-least the way
I see this issue. I may not have full context here, sorry.
This is correct, FW here does not actually prevent core from starting
since it is not a significant enough thermal issue.