linux-kernel - Re: [PATCH 7/7] mm/page_alloc: Replace local

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Message-ID: <04709b2d0dc702c9bf50f57cde125b07cdf54363.camel@redhat.com>
Date:   Fri, 17 Jun 2022 11:39:03 +0200
From:   Nicolas Saenz Julienne <nsaenzju@...hat.com>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 7/7] mm/page_alloc: Replace local_lock with normal
 spinlock

Hi Mel,

On Mon, 2022-06-13 at 13:56 +0100, Mel Gorman wrote:
> @@ -3446,12 +3490,16 @@ void free_unref_page(struct page *page, unsigned int order)
>  		migratetype = MIGRATE_MOVABLE;
>  	}
>  
> -	local_lock_irqsave(&pagesets.lock, flags);
> -	freed_pcp = free_unref_page_commit(page, migratetype, order, false);
> -	local_unlock_irqrestore(&pagesets.lock, flags);
> -
> -	if (unlikely(!freed_pcp))
> +	zone = page_zone(page);
> +	pcp_trylock_prepare(UP_flags);

Now that you're calling the *_irqsave() family of function you can drop
pcp_trylock_prepare/finish()

For the record in UP:

#define spin_trylock_irqsave(lock, flags) \
({ \
	local_irq_save(flags); \
	1;
})

> +	pcp = pcpu_spin_trylock_irqsave(struct per_cpu_pages, lock, zone->per_cpu_pageset, flags);
> +	if (pcp) {
> +		free_unref_page_commit(pcp, zone, page, migratetype, order);
> +		pcp_spin_unlock_irqrestore(pcp, flags);
> +	} else {
>  		free_one_page(page_zone(page), page, pfn, order, migratetype, FPI_NONE);
> +	}
> +	pcp_trylock_finish(UP_flags);
>  }
>  
>  /*

As Vlastimil mentioned elsewhere, I also wonder if it makes sense to just
bypass patch #5. Especially as its intent isn't true anymore:

"As preparation for dealing with both of those problems, protect the lists
with a spinlock.  The IRQ-unsafe version of the lock is used because IRQs
are already disabled by local_lock_irqsave.  spin_trylock is used in
preparation for a time when local_lock could be used instead of
lock_lock_irqsave."

-- 
Nicolás Sáenz