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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Mel Gorman <mgorman@techsingularity.net>,
        Vlastimil Babka <vbabka@suse.cz>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Hugh Dickins <hughd@google.com>, Linux-MM <linux-mm@kvack.org>,
        Linux-RT-Users <linux-rt-users@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 2/2] mm/vmstat: Protect per cpu variables with preempt
 disable on RT
In-Reply-To: <20210804142306.GE6464@techsingularity.net>
References: <20210723100034.13353-1-mgorman@techsingularity.net>
 <20210723100034.13353-3-mgorman@techsingularity.net> <87czqu2iew.ffs@tglx>
 <20210804095425.GA6464@techsingularity.net>
 <91b2f893-eb6a-d91d-3769-baba8601b0f6@suse.cz>
 <20210804142306.GE6464@techsingularity.net>
Date:   Thu, 05 Aug 2021 14:56:53 +0200
Message-ID: <87h7g4123u.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
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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.

Thanks,

        tglx