Re: [RFC PATCH 3/3] mm/memcg: Allow the task_obj optimization only on non-PREEMPTIBLE kernels.

On 12/22/21 06:41, Sebastian Andrzej Siewior wrote:

Based on my understanding the optimisation with task_obj for in_task()
mask sense on non-PREEMPTIBLE kernels because preempt_disable()/enable()
is optimized away. This could be then restricted to !CONFIG_PREEMPTION kernel
instead to only PREEMPT_RT.
With CONFIG_PREEMPT_DYNAMIC a non-PREEMPTIBLE kernel can also be
configured but these kernels always have preempt_disable()/enable()
present so it probably makes no sense here for the optimisation.

Restrict the optimisation to !CONFIG_PREEMPTION kernels.

Signed-off-by: Sebastian Andrzej Siewior <redacted>

If PREEMPT_DYNAMIC is selected, PREEMPTION will also be set. My 
understanding is that some distros are going to use PREEMPT_DYNAMIC, but 
default to PREEMPT_VOLUNTARY. So I don't believe it is a good idea to 
disable the optimization based on PREEMPTION alone.

Regards,
Longman