linux-kernel - Re: [PATCH 1/1] mm/vmstat: Protect per cpu variables with preempt disable on RT

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Date:   Fri, 6 Aug 2021 09:50:03 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>
Subject: Re: [PATCH 1/1] mm/vmstat: Protect per cpu variables with preempt
 disable on RT

On 8/6/21 1:22 AM, Andrew Morton wrote:
> On Thu,  5 Aug 2021 17:00:19 +0100 Mel Gorman <mgorman@...hsingularity.net> wrote:
> 
>> From: Ingo Molnar <mingo@...e.hu>
>> 
>> Disable preemption on -RT for the vmstat code. On vanila the code runs
>> in IRQ-off regions while on -RT it may not when stats are updated under
>> a local_lock. "preempt_disable" ensures that the same resources is not
>> updated in parallel due to preemption.
>> 
>> This patch differs from the preempt-rt version where __count_vm_event and
>> __count_vm_events are also protected. The counters are explicitly "allowed
>> to be to be racy" so there is no need to protect them from preemption. Only
>> the accurate page stats that are updated by a read-modify-write need
>> protection. This patch also differs in that a preempt_[en|dis]able_rt
>> helper is not used. As vmstat is the only user of the helper, it was
>> suggested that it be open-coded in vmstat.c instead of risking the helper
>> being used in unnecessary contexts.
>> 
>> ...
>>
>> --- a/mm/vmstat.c
>> +++ b/mm/vmstat.c
>> @@ -319,6 +319,16 @@ void __mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
>>  	long x;
>>  	long t;
>>  
>> +	/*
>> +	 * Accurate vmstat updates require a RMW. On !PREEMPT_RT kernels,
>> +	 * atomicity is provided by IRQs being disabled -- either explicitly
>> +	 * or via local_lock_irq. On PREEMPT_RT, local_lock_irq only disables
>> +	 * CPU migrations and preemption potentially corrupts a counter so
>> +	 * disable preemption.
>> +	 */
>> +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
>> +		preempt_disable();
> 
> This is so obvious I expect it has been discussed, but...  why not
> 
> static inline void preempt_disable_if_rt(void)
> {
> 	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> 		preempt_disable();
> }

Yeah v1 introduced similar more generic ones in include/linux/preempt.h and
Thomas didn't like that. I guess it would be more acceptable when confined to
mm/vmstat.c