Re: [RFC PATCH 0/1] sched/fair: Feature to suppress Fair Server for NOHZ_FULL isolation

[Daniel Vacek]

On Tue, 6 Jan 2026 at 16:38, Valentin Schneider <vschneid@xxxxxxxxxx> wrote:
> On 06/01/26 14:37, Shrikanth Hegde wrote:
> > On 1/6/26 9:12 AM, Aaron Tomlin wrote:
> >> Hi Ingo, Peter, Juri, Vincent,
> >>
> >> This patch introduces a new scheduler feature, RT_SUPPRESS_FAIR_SERVER,
> >> designed to ensure strict NOHZ_FULL isolation for SCHED_FIFO workloads,
> >> particularly in the presence of resident CFS tasks.
> >>
> >> In strictly partitioned, latency-critical environments (such as High
> >> Frequency Trading platforms) administrators frequently employ fully
> >> adaptive-tick CPUs to execute pinned SCHED_FIFO workloads. The fundamental
> >> requirement is "zero OS noise"; specifically, the scheduler clock-tick must
> >> remain suppressed ("offloaded"), given that standard SCHED_FIFO semantics
> >> dictate no forced preemption between tasks of identical priority.
> >
> > If all your SCHED_FIFO is pinned and their scheduling decisions
> > are managed in userspace, using isolcpus would offer you better
> > isolations compared to nohz_full.
> >
>
> Right, that's the part I don't get; why not use CPU isolation / cpusets to
> isolate the CPUs running those NOHZ_FULL applications? Regardless of the
> deadline server, if CFS tasks get scheduled on the same CPU as your
> latency-sensitive tasks then something's not right.

Some kernel workers and threaded interrupt handlers can be local/pinned, right?

For example this is usually (was often?) visible with DPDK
applications like FlexRAN/OpenRAN, etc.
And Aaron has mentioned high speed trading before.