Re: [PATCH v2 2/2] powerpc: Large user copy aware of full:rt:lazy preemption

On 11/20/24 13:33, Sebastian Andrzej Siewior wrote:

On 2024-11-19 13:08:31 [-0800], Ankur Arora wrote:

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Shrikanth Hegde [off-list ref] writes:

Thanks Ankur and Sebastian for taking a look.

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Large user copy_to/from (more than 16 bytes) uses vmx instructions to
speed things up. Once the copy is done, it makes sense to try schedule
as soon as possible for preemptible kernels. So do this for
preempt=full/lazy and rt kernel.

Note that this check will also fire for PREEMPT_DYNAMIC && preempt=none.
So when power supports PREEMPT_DYNAMIC this will need to change
to preempt_model_*() based checks.

Yes. This and return to kernel both needs to change when PowerPC support PREEMPT_DYNAMIC.
I have a patch in work in which I essentially do check for the preemption model.
Either below or based on static key.

-	if (IS_ENABLED(CONFIG_PREEMPTION) && need_resched())
+	if (preempt_model_preemptible() && need_resched())



+mark +valentin

More looking into how PREEMPPT_DYNAMIC works with static key, I have one query.
This is more on PREEMPT_DYNAMIC than anything to with LAZY.

I see many places use static_key based check instead of using preempt_model_preemptible such as
dynamic_preempt_schedule, is it because static_key is faster?

On the other hand, using preempt_model_preemptible could make the code simpler.

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Not checking for lazy bit here, since it could lead to unnecessary
context switches.

Maybe:
Not checking for lazy bit here, since we only want to schedule when
a context switch is imminently required.

Isn't his behaviour here exactly what preempt_enable() would do?
If the LAZY bit is set, it is delayed until return to userland or an
explicit schedule() because it is done. If this LAZY bit turned into an
actual scheduling request then it is acted upon.

Sebastian