Tickless kernel / Configure boot cpu?

smudge · 05-20-2017, 04:08 PM

Hi,
I'm trying to set up a tickless kernel, and I would like CPU1 to be my maintenance CPU. However, CPU0, keeps getting removed from the nohz_full range. I added nohz_full=0,2-15 to my the kernel startup arguments. I see this in the dmesg:

Code:

[    0.000000] NO_HZ: Clearing 0 from nohz_full range for timekeeping

I can confirm every cpu except 0 and 1 are in tickless mode. According to the kernel documentation:

Code:

nohz_full=      [KNL,BOOT]
                        In kernels built with CONFIG_NO_HZ_FULL=y, set
                        the specified list of CPUs whose tick will be stopped
                        whenever possible. The boot CPU will be forced outside
                        the range to maintain the timekeeping.
                        The CPUs in this range must also be included in the
                        rcu_nocbs= set.

The nohz_full should have set the boot CPU outside the range, which is CPU1. Instead it cleared 0 from the nohz_full range. Is there a way to force the boot cpu?

I'm using kernel 3.10.

syg00 · 05-20-2017, 05:31 PM

I would doubt it. Chicken-and-egg scenario - the kernel has to already be active to parse the parms you passed it.
Back when I last was interested enough to investigate this, the kernel always loaded in single-CPU mode, then checked for and activated additional processors. Possibly still the case.