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

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Date:   Thu, 05 Aug 2021 17:42:40 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Hugh Dickins <hughd@...gle.com>, Linux-MM <linux-mm@...ck.org>,
        Linux-RT-Users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] mm/vmstat: Protect per cpu variables with preempt
 disable on RT

On Thu, Aug 05 2021 at 15:04, Mel Gorman wrote:
> 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?

Looks good.

Acked-by: Thomas Gleixner <tglx@...utronix.de>