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From: Peter Zijlstra <peterz@infradead.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Linux-MM <linux-mm@kvack.org>,
	Linux-RT-Users <linux-rt-users@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Chuck Lever <chuck.lever@oracle.com>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>, Michal Hocko <mhocko@kernel.org>,
	Oscar Salvador <osalvador@suse.de>
Subject: Re: [PATCH 02/11] mm/page_alloc: Convert per-cpu list protection to local_lock
Date: Fri, 9 Apr 2021 08:39:45 +0200
Message-ID: <YG/2scd9ADdrIyCM@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210408174244.GG3697@techsingularity.net>

On Thu, Apr 08, 2021 at 06:42:44PM +0100, Mel Gorman wrote:
> 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.

Fair enough; I was just wondering why the obvious solution wasn't chosen
and neither changelog nor comment explain, so I had to ask :-)

  reply index

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-07 20:24 [PATCH 0/11 v2] Use local_lock for pcp protection and reduce stat overhead Mel Gorman
2021-04-07 20:24 ` [PATCH 01/11] mm/page_alloc: Split per cpu page lists and zone stats Mel Gorman
2021-04-12 17:43   ` Vlastimil Babka
2021-04-13 13:27     ` Mel Gorman
2021-04-07 20:24 ` [PATCH 02/11] mm/page_alloc: Convert per-cpu list protection to local_lock Mel Gorman
2021-04-08 10:52   ` Peter Zijlstra
2021-04-08 17:42     ` Mel Gorman
2021-04-09  6:39       ` Peter Zijlstra [this message]
2021-04-09  7:59         ` Mel Gorman
2021-04-09  8:24           ` Peter Zijlstra
2021-04-09 13:32             ` Mel Gorman
2021-04-09 18:55               ` Peter Zijlstra
2021-04-12 11:56                 ` Mel Gorman
2021-04-12 21:47                   ` Thomas Gleixner
2021-04-13 16:52                     ` Mel Gorman
2021-04-07 20:24 ` [PATCH 03/11] mm/memory_hotplug: Make unpopulated zones PCP structures unreachable during hot remove Mel Gorman
2021-04-07 20:24 ` [PATCH 04/11] mm/vmstat: Convert NUMA statistics to basic NUMA counters Mel Gorman
2021-04-14 12:56   ` Vlastimil Babka
2021-04-14 15:18     ` Mel Gorman
2021-04-14 15:56       ` Vlastimil Babka
2021-04-15 10:06         ` Mel Gorman
2021-04-07 20:24 ` [PATCH 05/11] mm/vmstat: Inline NUMA event counter updates Mel Gorman
2021-04-07 20:24 ` [PATCH 06/11] mm/page_alloc: Batch the accounting updates in the bulk allocator Mel Gorman
2021-04-07 20:24 ` [PATCH 07/11] mm/page_alloc: Reduce duration that IRQs are disabled for VM counters Mel Gorman
2021-04-07 20:24 ` [PATCH 08/11] mm/page_alloc: Remove duplicate checks if migratetype should be isolated Mel Gorman
2021-04-07 20:24 ` [PATCH 09/11] mm/page_alloc: Explicitly acquire the zone lock in __free_pages_ok Mel Gorman
2021-04-07 20:24 ` [PATCH 10/11] mm/page_alloc: Avoid conflating IRQs disabled with zone->lock Mel Gorman
2021-04-07 20:24 ` [PATCH 11/11] mm/page_alloc: Update PGFREE outside the zone lock in __free_pages_ok Mel Gorman
2021-04-08 10:56 ` [PATCH 0/11 v2] Use local_lock for pcp protection and reduce stat overhead Peter Zijlstra
2021-04-08 17:48   ` Mel Gorman
2021-04-14 13:39 [PATCH 0/11 v3] " Mel Gorman
2021-04-14 13:39 ` [PATCH 02/11] mm/page_alloc: Convert per-cpu list protection to local_lock Mel Gorman

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