Re: [RESEND PATCH] tick/nohz: Fix wrong NOHZ idle CPU state

[Shubhang Kaushik <shubhang@os.amperecomputing.com>]

On Thu, 12 Feb 2026, Shubhang Kaushik wrote:

>>  Because you rely on dynamic placement of isolated tasks throughout
>>  isolated
>>  CPUs by the scheduler.
>>
>>  But nohz_full is designed for running only one task per isolated CPU
>>  without
>>  any disturbance. And migration is a significant disturbance. This is why
>>  nohz_full tries not to be too smart and assumes that task placement is
>>  entirely
>>  within the hands of the user.
>>
>>  So I have to ask, what prevents you from using static task placement in
>>  your
>>  workload?
>
> Actually, the llama-batched-bench results I shared already included static 
> affinity testing via numactl -C.

What I mean by that is even when tasks are strictly pinned to individual 
cores, the performance gap remains.

IIUC, the current implementation assumes tick-stop and idle-entry are 
coupled. While this holds for standard NOHZ, nohz_full decouples them, 
causing idle CPUs to be omitted from nohz.idle_cpus_mask.

This hides idle capacity from  the NOHZ idle balancer, forcing 
housekeeping tasks onto active cores. By decoupling these transitions in 
the code, we ensure accurate state accounting.

>
> Even with static placement, we observe this ~14% throughput improvement. This 
> suggests that the issue isn't about the scheduler trying to be smart with 
> task migration, but rather about the side effects of an idle CPU being absent 
> from nohz.idle_cpus_mask.
>
> When nohz_full CPUs enter idle but aren't correctly accounted for in the idle 
> mask, it appears to cause unnecessary overhead or interference in the NOHZ 
> load balancing logic for the CPUs that are still running tasks. By ensuring 
> the idle state is correctly tracked, we're not encouraging migration, but 
> rather ensuring the scheduler's global state accurately reflects reality.
>
> AFAICT this seems to be a case where correcting the bookkeeping benefits HPC 
> throughput even when the user handles all task placement manually.
>
> Regards,
> Shubhang Kaushik
>>
>>  I'm not saying it's undesirable or impossible to do adaptive userspace
>>  dyntick
>>  for users that don't rely on ultra low latency but rather on high
>>  CPU-bound
>>  performance. In fact the initial purpose of nohz_full was for HPC and not
>>  real-time. Turns out that real time is all the usecase I have seen so far
>>  and
>>  you're the first HPC one. But adapting nohz_full dynamically for that will
>>  involve
>>  much more than just load balancing. Now the static affinity should work
>>  for
>>  everyone.
>>
>>  Thanks.
>> 
>> 
>>>
>>>  Signed-off-by: Shubhang Kaushik <shubhang@os.amperecomputing.com>
>>>  Signed-off-by: Adam Li <adamli@os.amperecomputing.com>
>>>  Reviewed-by: Christoph Lameter (Ampere) <cl@gentwo.org>
>>>  Reviewed-by: Shubhang Kaushik <shubhang@os.amperecomputing.com>
>>>  ---
>>>  This is a resend of the original patch to ensure visibility.
>>>  Previous resend: https://lkml.org/lkml/2025/8/21/170
>>>  Original thread: https://lkml.org/lkml/2025/8/21/171
>>>
>>>  The patch addresses a performance regression in NOHZ idle load balancing
>>>  observed under CONFIG_NO_HZ_FULL, where idle CPUs were becoming
>>>  invisible to the balancer.
>>>  ---
>>>   kernel/time/tick-sched.c | 5 +++--
>>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>>  diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
>>>  index
>>>  2f8a7923fa279409ffe950f770ff2eac868f6ece..eee6fcebe78c2f8d93464a55fe332e12fe9c164e
>>>  100644
>>>  --- a/kernel/time/tick-sched.c
>>>  +++ b/kernel/time/tick-sched.c
>>>  @@ -1250,8 +1250,9 @@ void tick_nohz_idle_stop_tick(void)
>>>     ts->idle_sleeps++;
>>>     ts->idle_expires = expires;
>>>
>>>  -		if (!was_stopped && tick_sched_flag_test(ts,
>>>  TS_FLAG_STOPPED)) {
>>>  -			ts->idle_jiffies = ts->last_jiffies;
>>>  +		if (tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
>>>  +			if (!was_stopped)
>>>  +				ts->idle_jiffies = ts->last_jiffies;
>>>     	nohz_balance_enter_idle(cpu);
>>>    	}
>>>    } else {
>>>
>>>  ---
>>>  base-commit: 18f7fcd5e69a04df57b563360b88be72471d6b62
>>>  change-id: 20260203-fix-nohz-idle-b2838276cb91
>>>
>>>  Best regards,
>>>  --
>>>  Shubhang Kaushik <shubhang@os.amperecomputing.com>
>>> 
>>
>>  --
>>  Frederic Weisbecker
>>  SUSE Labs
>> 
>
>