Re: [PATCH 2/2] mm/vmstat: Protect per cpu variables with preempt disable on RT

[Mel Gorman]

On Thu, Aug 05, 2021 at 02:56:53PM +0200, Thomas Gleixner wrote:
> On Wed, Aug 04 2021 at 15:23, Mel Gorman wrote:
> Mel,
> 
> > On Wed, Aug 04, 2021 at 03:42:25PM +0200, Vlastimil Babka wrote:
> >> The idea was not build-time, but runtime (hidden behind lockdep, VM_DEBUG or
> >> whatnot), i.e.:
> >> 
> >> <sched_expert> what that code needs is switch(item) { case foo1: case foo2:
> >> lockdep_assert_irqs_disabled(); break; case bar1: case bar2:
> >> lockdep_assert_preempt_disabled(); lockdep_assert_no_in_irq(); break; } or
> >> something along those lines
> >> 
> > Ok, that would potentially work. It may not even need to split the stats
> > into different enums. Simply document which stats need protection from
> > IRQ or preemption and use PROVE_LOCKING to check if preemption or IRQs
> > are disabled depending on the kernel config. I don't think it gets rid
> > of preempt_disable_rt unless the API was completely reworked with entry
> > points that describe the locking requirements. That would be tricky
> > because the requirements differ between kernel configurations.
> 
> Right. This won't get rid of the preempt disabling on RT, but I think we
> should rather open code this
> 
>        if (IS_ENABLED(CONFIG_PREEMPT_RT))
>        		preempt_dis/enable();
> 
> instead of proliferating these helper macros which have only one user left.
> 

Ok, that is reasonable. I tried creating a vmstat-specific helper but the
names were misleading so I ended up with the patch below which open-codes
it as you suggest. The comment is not accurate because "locking/local_lock:
Add RT support" is not upstream but it'll eventually be accurate.

Is this ok?

--8<--