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

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Message-ID: <20210408174244.GG3697@techsingularity.net>
Date:   Thu, 8 Apr 2021 18:42:44 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux-RT-Users <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH 02/11] mm/page_alloc: Convert per-cpu list protection to
 local_lock

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