[PATCH 0/1][N/U] Enable lowlatency settings in the generic kernel

[Andrea Righi]

On Fri, Jan 26, 2024 at 10:23:32AM -0800, Jay Vosburgh wrote:
> Andrea Righi <andrea.righi at canonical.com> wrote:
> 
> [...]
> >- CONFIG_NO_HZ_FULL=y: enable access to "Full tickless mode" (shutdown
> >  clock tick when possible across all the enabled CPUs if they are
> >  either idle or running 1 task - reduce kernel jitter of running tasks
> >  due to the periodic clock tick, must be enabled at boot time passing
> >  `nohz_full=<cpu_list>`); this can actually help CPU-intensive
> >  workloads and it could provide much more benefits than the CONFIG_HZ
> >  difference (since it can potentially shutdown any kernel jitter on
> >  specific CPUs), this one should really be enabled anyway, considering
> >  that it is configurable at boot time
> 
> 	FYI, we did some testing with CONFIG_NO_HZ_FULL=y as part of LP
> 1919154 [0].  The testing timed context switches, and did find some
> additional overhead on AMD EPYC when NO_HZ_FULL is configured but not
> enabled at boot.  This was on the Ubuntu 5.15 kernel, so, not the most
> recent, but something to be aware of.  The test program is attached to
> the bug.
> 
> 	-J
> 
> [0] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1919154

Thanks Jay, I'll repeat the same test with the 6.8 kernel and NO_HZ_FULL
disabled/enabled.

I have an AMD (not the same exact architecture) and the kernel has
changed a lot from 5.15, but I'll give it a try anyway.

-Andrea