Re: 4.2: CONFIG_NO_HZ_FULL_ALL effectively disabling non-boot CPUs

[Paul E. McKenney]

On Sat, Oct 10, 2015 at 10:14:25PM +0300, Meelis Roos wrote:
> Short summary: turning on CONFIG_NO_HZ_FULL_ALL seems to disable all 
> non-boot CPUs for scheduler.
> 
> A couple of days ago I noticed that make -j8 on a 4-core i5 is very slow 
> (with 4.3.0-rc4+git). Looking at top ('1' for per-cpu states), only 
> first CPU is loaded and 3 other CPUs are 100% idle. This seems to be a 
> problem on 3 of my desktop machines (different generation Intel: i5-660, 
> i5-2400, i3-3220). All the computers run custom kernels.
> 
> Further investigation showed that CPU affinity was set to 1 (CPU0 only) 
> for init and all the children. Kernel threads had affinities 1,2,4,8 
> and f (seems normal).
> 
> Even more interesting was the behaviour after setting affinity to f for 
> all userland processes and then running make -j4. The other cores were 
> still idle!
> 
> Switching back to 4.2.0 with my config, the problem persisted. 4.2.3 as 
> packaged by Debian worked fine. 4.0.0 and 4.1.0 with my config worked 
> also fine. systemd and sysvinit behaved the same and no affinity was 
> configured for systemd.
> 
> So did a kernel config bisection between my kernel config and Debian 
> config and came to CONFIG_NO_HZ_FULL_ALL. Debian has it off, I had it 
> on. Turning that off fixed the scheduling and the system spread the 
> tasks to all the cores.
> 
> I do not remember changing this value for a long time, I set them after 
> the settings were introduced and used it. So it seems it broken in 4.2.0 
> but was working in 4.1 but I do not have 4.1 config saved anywhere 
> (many make oldconfigs since).
> 
> Bisection between 4.1 and 4.2 is possible but not easy since the 
> machines are usually actively used when I am near them.

This is expected and intended behavior.  The whole point of
CONFIG_NO_HZ_FULL_ALL is to keep everything off of the non-boot CPUs
that is not explicitly placed there.  Without CONFIG_NO_HZ_FULL_ALL,
you can use the nohz_full boot parameter to select exactly which
CPUs are to behave this way.

							Thanx, Paul

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