Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

Ankur Arora
2023-10-18T20:16:08Z

Paul E. McKenney <paulmck@kernel.org> writes:

> On Wed, Oct 18, 2023 at 03:16:12PM +0200, Thomas Gleixner wrote:
>> Paul!
>>
>> On Tue, Oct 17 2023 at 18:03, Paul E. McKenney wrote:
>> > Belatedly calling out some RCU issues. Nothing fatal, just a
>> > (surprisingly) few adjustments that will need to be made. The key thing
>> > to note is that from RCU's viewpoint, with this change, all kernels
>> > are preemptible, though rcu_read_lock() readers remain
>> > non-preemptible.
>>
>> Why? Either I'm confused or you or both of us :)
>
> Isn't rcu_read_lock() defined as preempt_disable() and rcu_read_unlock()
> as preempt_enable() in this approach? I certainly hope so, as RCU
> priority boosting would be a most unwelcome addition to many datacenter
> workloads.

No, in this approach, PREEMPT_AUTO selects PREEMPTION and thus
PREEMPT_RCU so rcu_read_lock/unlock() would touch the
rcu_read_lock_nesting. Which is identical to what PREEMPT_DYNAMIC does.

>> With this approach the kernel is by definition fully preemptible, which
>> means means rcu_read_lock() is preemptible too. That's pretty much the
>> same situation as with PREEMPT_DYNAMIC.
>
> Please, just no!!!
>
> Please note that the current use of PREEMPT_DYNAMIC with preempt=none
> avoids preempting RCU read-side critical sections. This means that the
> distro use of PREEMPT_DYNAMIC has most definitely *not* tested preemption
> of RCU readers in environments expecting no preemption.

Ah. So, though PREEMPT_DYNAMIC with preempt=none runs with PREEMPT_RCU,
preempt=none stubs out the actual preemption via __preempt_schedule.

Okay, I see what you are saying.

(Side issue: but this means that even for PREEMPT_DYNAMIC preempt=none,
_cond_resched() doesn't call rcu_all_qs().)

>> For throughput sake this fully preemptible kernel provides a mechanism
>> to delay preemption for SCHED_OTHER tasks, i.e. instead of setting
>> NEED_RESCHED the scheduler sets NEED_RESCHED_LAZY.
>>
>> That means the preemption points in preempt_enable() and return from
>> interrupt to kernel will not see NEED_RESCHED and the tasks can run to
>> completion either to the point where they call schedule() or when they
>> return to user space. That's pretty much what PREEMPT_NONE does today.
>>
>> The difference to NONE/VOLUNTARY is that the explicit cond_resched()
>> points are not longer required because the scheduler can preempt the
>> long running task by setting NEED_RESCHED instead.
>>
>> That preemption might be suboptimal in some cases compared to
>> cond_resched(), but from my initial experimentation that's not really an
>> issue.
>
> I am not (repeat NOT) arguing for keeping cond_resched(). I am instead
> arguing that the less-preemptible variants of the kernel should continue
> to avoid preempting RCU read-side critical sections.

[ snip ]

>> In the end there is no CONFIG_PREEMPT_XXX anymore. The only knob
>> remaining would be CONFIG_PREEMPT_RT, which should be renamed to
>> CONFIG_RT or such as it does not really change the preemption
>> model itself. RT just reduces the preemption disabled sections with the
>> lock conversions, forced interrupt threading and some more.
>
> Again, please, no.
>
> There are situations where we still need rcu_read_lock() and
> rcu_read_unlock() to be preempt_disable() and preempt_enable(),
> repectively. Those can be cases selected only by Kconfig option, not
> available in kernels compiled with CONFIG_PREEMPT_DYNAMIC=y.

As far as non-preemptible RCU read-side critical sections are concerned,
are the current
- PREEMPT_DYNAMIC=y, PREEMPT_RCU, preempt=none config
(rcu_read_lock/unlock() do not manipulate preempt_count, but do
stub out preempt_schedule())
- and PREEMPT_NONE=y, TREE_RCU config (rcu_read_lock/unlock() manipulate
preempt_count)?

roughly similar or no?

>> > I am sure that I am missing something, but I have not yet seen any
>> > show-stoppers. Just some needed adjustments.
>>
>> Right. If it works out as I think it can work out the main adjustments
>> are to remove a large amount of #ifdef maze and related gunk :)
>
> Just please don't remove the #ifdef gunk that is still needed!

Always the hard part :).

Thanks

--
ankur