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Date:   Fri, 28 May 2021 10:49:49 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     David Hildenbrand <david@redhat.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hillf Danton <hdanton@sina.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, "Tang, Feng" <feng.tang@intel.com>
Subject: Re: [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and
 active CPUs
Message-ID: <20210528094949.GL30378@techsingularity.net>
References: <20210525080119.5455-1-mgorman@techsingularity.net>
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 <20210528085545.GJ30378@techsingularity.net>
 <54ff0363-2f39-71d1-e26c-962c3fddedae@redhat.com>
 <e73d39cc-df2f-5000-0212-6073955fb291@redhat.com>
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On Fri, May 28, 2021 at 11:08:01AM +0200, David Hildenbrand wrote:
> On 28.05.21 11:03, David Hildenbrand wrote:
> > On 28.05.21 10:55, Mel Gorman wrote:
> > > On Thu, May 27, 2021 at 12:36:21PM -0700, Dave Hansen wrote:
> > > > Hi Mel,
> > > > 
> > > > Feng Tang tossed these on a "Cascade Lake" system with 96 threads and
> > > > ~512G of persistent memory and 128G of DRAM.  The PMEM is in "volatile
> > > > use" mode and being managed via the buddy just like the normal RAM.
> > > > 
> > > > The PMEM zones are big ones:
> > > > 
> > > >           present  65011712 = 248 G
> > > >           high       134595 = 525 M
> > > > 
> > > > The PMEM nodes, of course, don't have any CPUs in them.
> > > > 
> > > > With your series, the pcp->high value per-cpu is 69584 pages or about
> > > > 270MB per CPU.  Scaled up by the 96 CPU threads, that's ~26GB of
> > > > worst-case memory in the pcps per zone, or roughly 10% of the size of
> > > > the zone.
> > 
> > When I read about having such big amounts of free memory theoretically
> > stuck in PCP lists, I guess we really want to start draining the PCP in
> > alloc_contig_range(), just as we do with memory hotunplug when offlining.
> > 
> 
> Correction: we already drain the pcp, we just don't temporarily disable it,
> so a race as described in offline_pages() could apply:
> 
> "Disable pcplists so that page isolation cannot race with freeing
>  in a way that pages from isolated pageblock are left on pcplists."
> 
> Guess we'd then want to move the draining before start_isolate_page_range()
> in alloc_contig_range().
> 

Or instead of draining, validate the PFN range in alloc_contig_range
is within the same zone and if so, call zone_pcp_disable() before
start_isolate_page_range and enable after __alloc_contig_migrate_range.

-- 
Mel Gorman
SUSE Labs