On 1/3/22 09:44, Sebastian Andrzej Siewior wrote:
On 2021-12-23 16:48:41 [-0500], Waiman Long wrote:
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 <bigeasy@xxxxxxxxxxxxx>
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.
So there is a benefit to this even if preempt_disable() is not optimized
away? My understanding was that this depends on preempt_disable() being
optimized away.
Is there something you recommend as a benchmark where I could get some
numbers?
In the case of PREEMPT_DYNAMIC, it depends on the default setting which
is used by most users. I will support disabling the optimization if
defined(CONFIG_PREEMPT_RT) || defined(CONFIG_PREEMPT), just not by
CONFIG_)PREEMPTION alone.
As for microbenchmark, something that makes a lot of calls to malloc()
or related allocations can be used.
Cheers,
Longman
