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Subject: Re: [PATCH 1/1] mm/vmstat: Protect per cpu variables with preempt
 disable on RT
To:     Mel Gorman <mgorman@techsingularity.net>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        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>
References: <20210805160019.1137-1-mgorman@techsingularity.net>
 <20210805160019.1137-2-mgorman@techsingularity.net>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <d54e0958-5680-0d5c-e4a7-6ec4afe53b48@suse.cz>
Date:   Fri, 6 Aug 2021 14:38:54 +0200
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On 8/5/21 6:00 PM, Mel Gorman wrote:
> From: Ingo Molnar <mingo@elte.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.
> 
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Acked-by: Vlastimil Babka <vbabka@suse.cz>