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

[Ankur Arora]

Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:

> Paul!
>
> On Tue, Nov 21 2023 at 07:19, Paul E. McKenney wrote:
>> On Tue, Nov 21, 2023 at 10:00:59AM -0500, Steven Rostedt wrote:
>>> Right now, the use of cond_resched() is basically a whack-a-mole game where
>>> we need to whack all the mole loops with the cond_resched() hammer. As
>>> Thomas said, this is backwards. It makes more sense to just not preempt in
>>> areas that can cause pain (like holding a mutex or in an RCU critical
>>> section), but still have the general kernel be fully preemptable.
>>
>> Which is quite true, but that whack-a-mole game can be ended without
>> getting rid of build-time selection of the preemption model.  Also,
>> that whack-a-mole game can be ended without eliminating all calls to
>> cond_resched().
>
> Which calls to cond_resched() should not be eliminated?
>
> They all suck and keeping some of them is just counterproductive as
> again people will sprinkle them all over the place for the very wrong
> reasons.

And, as Thomas alludes to here, cond_resched() is not always cost free.
Needing to call cond_resched() forces us to restructure hot paths in
ways that results in worse performance/complex code.

One example is clear_huge_page(), where removing the need to call
cond_resched() every once in a while allows the processor to optimize
differently.

  *Milan*     mm/clear_huge_page   x86/clear_huge_page   change
                          (GB/s)                (GB/s)

  pg-sz=2MB                14.55                 19.29    +32.5%
  pg-sz=1GB                19.34                 49.60   +156.4%

(See https://lore.kernel.org/all/20230830184958.2333078-1-ankur.a.arora@xxxxxxxxxx/)

And, that's one of the simpler examples from mm. We do this kind of arbitrary
batching all over the place.

Or see the filemap_read() example that Linus gives here:
 https://lore.kernel.org/lkml/CAHk-=whpYjm_AizQij6XEfTd7xvGjrVCx5gzHcHm=2Xijt+Kyg@xxxxxxxxxxxxxx/#t

Thanks
--
ankur