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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

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.

> 
> However, the extant "Fair Server" (Deadline Server) architecture
> compromises this isolation guarantee. At present, should a background
> SCHED_OTHER task be enqueued, the scheduler initiates the Fair Server
> (dl_server_start). As the Fair Server functions as a SCHED_DEADLINE entity,
> its activation increments rq->dl.dl_nr_running.
> 

There is runtime allocated to fair server. If you make them 0 on CPUs of
interest, wouldn't that work?

/sys/kernel/debug/sched/fair_server/<cpu>/runtime

> This condition compels sched_can_stop_tick() to return false, thereby
> restarting the periodic tick to enforce the server's runtime.
> To address this, the patch introduces a new scheduler feature control,
> RT_SUPPRESS_FAIR_SERVER.
> 
> When engaged, this modification amends enqueue_task_fair() to forego the
> invocation of dl_server_start() if, and only if, the following conditions
> are met:
> 
> 	1. A Real-Time task (SCHED_FIFO/SCHED_RR) is currently in execution
> 	2. RT bandwidth enforcement (rt_bandwidth_enabled()) is inactive
> 
> By precluding the server's initiation, rq->dl.dl_nr_running is maintained
> at zero. This permits the tick logic to defer to the standard SCHED_FIFO
> protocol, thereby ensuring the tick remains suppressed.
> 
> Considerations: This serves as a precision instrument for specialised
> contexts. It explicitly prioritises determinism over fairness. Whilst
> enabled, queued CFS tasks shall endure total starvation until such time as
> the RT task voluntarily yields. I believe this is acceptable for
> partitioned architectures where housekeeping duties are allocated to
> alternative cores; however, I have guarded this capability within
> CONFIG_NO_HZ_FULL and a default-disabled feature flag to obviate the risk
> of inadvertent starvation on general-purpose systems.
> 
> I welcome your thoughts on this approach.
> 
> 
> Aaron Tomlin (1):
>    sched/fair: Introduce RT_SUPPRESS_FAIR_SERVER to optimise NOHZ_FULL
>      isolation
> 
>   kernel/sched/fair.c     | 19 ++++++++++++++++++-
>   kernel/sched/features.h |  9 +++++++++
>   2 files changed, 27 insertions(+), 1 deletion(-)
>