Re: [PATCH 02/11] mm/page_alloc: Convert per-cpu list protection to local_lock

[Mel Gorman <mgorman@techsingularity.net>]

On Thu, Apr 08, 2021 at 12:52:07PM +0200, Peter Zijlstra wrote:
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index a68bacddcae0..e9e60d1a85d4 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -112,6 +112,13 @@ typedef int __bitwise fpi_t;
> >  static DEFINE_MUTEX(pcp_batch_high_lock);
> >  #define MIN_PERCPU_PAGELIST_FRACTION	(8)
> >  
> > +struct pagesets {
> > +	local_lock_t lock;
> > +};
> > +static DEFINE_PER_CPU(struct pagesets, pagesets) = {
> > +	.lock = INIT_LOCAL_LOCK(lock),
> > +};
> 
> So why isn't the local_lock_t in struct per_cpu_pages ? That seems to be
> the actual object that is protected by it and is already per-cpu.
> 
> Is that because you want to avoid the duplication across zones? Is that
> worth the effort?

When I wrote the patch, the problem was that zone_pcp_reset freed the
per_cpu_pages structure and it was "protected" by local_irq_save(). If
that was converted to local_lock_irq then the structure containing the
lock is freed before it is released which is obviously bad.

Much later when trying to make the allocator RT-safe in general, I realised
that locking was broken and fixed it in patch 3 of this series. With that,
the local_lock could potentially be embedded within per_cpu_pages safely
at the end of this series.

-- 
Mel Gorman
SUSE Labs