linux-kernel - Re: [PATCH 5/6] mm/page_alloc: Protect PCP lists with a spinlock

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Message-ID: <d3ac62ae-740a-5261-2762-7d8120fb8fd2@suse.cz>
Date:   Tue, 26 Apr 2022 18:48:52 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 5/6] mm/page_alloc: Protect PCP lists with a spinlock

On 4/26/22 18:42, Nicolas Saenz Julienne wrote:
> On Wed, 2022-04-20 at 10:59 +0100, Mel Gorman wrote:
>> @@ -3082,15 +3093,22 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
>>   */
>>  void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
>>  {
>> -	unsigned long flags;
>>  	int to_drain, batch;
>>  
>> -	local_lock_irqsave(&pagesets.lock, flags);
>>  	batch = READ_ONCE(pcp->batch);
>>  	to_drain = min(pcp->count, batch);
>> -	if (to_drain > 0)
>> +	if (to_drain > 0) {
>> +		unsigned long flags;
>> +
>> +		/* free_pcppages_bulk expects IRQs disabled for zone->lock */
>> +		local_irq_save(flags);
> 
> Why dropping the local_lock? That approach is nicer to RT builds, and I don't
> think it makes a difference from a non-RT perspective.

I think the separate irq_disable+spin_lock here is actually broken on RT
config, as explained in Documentation/locking/locktypes.rst. pcp->lock would
have to be a raw_spin_lock.

> That said, IIUC, this will eventually disappear with subsequent patches, right?

So it wouldn't be mergeable even as a temporary step.

> 
>> +
>> +		spin_lock(&pcp->lock);
>>  		free_pcppages_bulk(zone, to_drain, pcp, 0);
>> -	local_unlock_irqrestore(&pagesets.lock, flags);
>> +		spin_unlock(&pcp->lock);
>> +
>> +		local_irq_restore(flags);
>> +	}
>>  }
>>  #endif
>>