Re: Kernel 6.14.11 dl_server_timer(...) causing IPI/Function Call Interrupts on isolcpu/nohz_full cores, performance regression

[Cao Ruichuang]

Hi Juri,

I tested the "sched/deadline: Make dl-server nohz full aware" change
shape from f237e524f3c7 on the current mainline tree in a minimal QEMU
setup.

I used a latest-tree tiny x86 kernel with:

  nohz_full=1 isolcpus=domain,managed_irq,1 irqaffinity=0 rcu_nocbs=1

and a CPU1-pinned busy loop as the only user workload. To observe the
periodic activity, I installed a kprobe on start_dl_timer and counted
hits over the same 5 second window.

On the unmodified tree, I consistently saw:

  START_DL_TIMER_COUNT=118

With the sched_can_stop_tick() change from f237e524f3c7 applied, I saw:

  START_DL_TIMER_COUNT=20
  START_DL_TIMER_COUNT=22

So on current mainline this still looks like a real improvement in the
same direction David reported earlier: the periodic dl-server activity
is reduced substantially, although it is not eliminated completely in my
QEMU setup.

I am not sending this as a patch, only as an extra data point in favor
of that fix direction on a latest-tree test setup.

Thanks,
Cao Ruichuang