Re: [RFC PATCH 47/86] rcu: select PREEMPT_RCU if PREEMPT

[Ankur Arora]

Paul E. McKenney <paulmck@xxxxxxxxxx> writes:

> On Mon, Nov 20, 2023 at 10:43:56PM -0500, Steven Rostedt wrote:
>> On Mon, 20 Nov 2023 16:28:50 -0800
>> "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
>>
>> > On Tue, Nov 07, 2023 at 07:27:03PM -0500, Steven Rostedt wrote:
>> > > On Tue,  7 Nov 2023 13:57:33 -0800
>> > > Ankur Arora <ankur.a.arora@xxxxxxxxxx> wrote:
>> > >
>> With the new preemption model, NONE, VOLUNTARY and PREEMPT are now going to
>> determine when NEED_RESCHED is set as supposed to NEED_RESCHED_LAZY. As
>> NEED_RESCHED_LAZY only schedules at kernel / user space transaction, and
>> NEED_RESCHED will schedule when possible (non-preempt disable section).
>
> So NONE really is still supposed to be there.  ;-)
>
>>  Key: L - NEED_RESCHED_LAZY - schedule only at kernel/user boundary
>>       N - NEED_RESCHED - schedule whenever possible (like PREEMPT does today)
>>
>> 			SCHED_OTHER	REAL-TIME/DL
>> 			  Schedule	  Schedule
>>
>> NONE:		      L		     L
>>
>> VOLUNTARY:		      L		     N
>>
>> PREEMPT:		      N		     N
>>
>>
>> So on NONE, NEED_RESCHED_LAZY is set only on scheduling SCHED_OTHER and RT.
>> Which means, it will not schedule until it goes into user space (*).
>>
>> On VOLUNTARY, NEED_RESCHED is set on RT/DL tasks, and LAZY on SCHED_OTHER.
>> So that RT and DL get scheduled just like PREEMPT does today.
>>
>> On PREEMPT, NEED_RESCHED is always set on all scheduling.
>>
>> (*) - caveat - After the next tick, if NEED_RESCHED_LAZY is set, then
>> NEED_RESCHED will be set and the kernel will schedule at the next available
>> moment, this is true for all three models!
>
> OK, so I see that this is now a SCHED_FEAT, and is initialized based
> on CONFIG_PREEMPT_* in kernel/sched/feature.h.  Huh.  OK, we can still
> control this at build time, which is fine.  I don't see how to set it
> at boot time, only at build time or from debugfs.  I will let those who
> want to set this at boot time complain, should they choose to do so.

Needless to say, these patches were just an RFC and so they both overreach
and also miss out things.

v1 adds a new preemption model which coexists with the current
CONFIG_PREEMPT_* and is broadly based on Thomas' PoC and is on the lines
we have been discussing.

>> There may be more details to work out, but the above is basically the gist
>> of the idea. Now, what do you want to do with RCU_PREEMPT? At run time, we
>> can go from NONE to PREEMPT full! But there may be use cases that do not
>> want the overhead of always having RCU_PREEMPT, and will want RCU to be a
>> preempt_disable() section no matter what.
>
> Understood, actually.  And as noted in other replies, I am a bit concerned
> about added latencies from too aggressively removing cond_resched().
>
> More testing required.

Yes, agreed.

--
ankur