Re: [PATCH 7/7] mm/page_alloc: Replace local_lock with normal spinlock

Nicolas Saenz Julienne
2022-06-17T09:39:21Z

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