linux-kernel - Re: [PATCH v2 1/1] mm: buddy page accessed before initialized

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Date:   Fri, 3 Nov 2017 14:53:49 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     Steve Sistare <steven.sistare@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        mgorman@...hsingularity.net,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] mm: buddy page accessed before initialized

On Fri 03-11-17 09:47:30, Pavel Tatashin wrote:
> Hi Michal,
> 
> There is a small regression, on the largest x86 machine I have access to:
> Before:
> node 1 initialised, 32471632 pages in 901ms
> After:
> node 1 initialised, 32471632 pages in 1128ms
> 
> One node contains 128G of memory (overal 1T in 8 nodes). This
> regression is going to be solved by this work:
> https://patchwork.kernel.org/patch/9920953/, other than that I do not
> know a better solution. The overall performance is still much better
> compared to before this project.

OK, I think that is completely acceptable for now. We can always
optimize for a better result later.

> Also, thinking about this problem some more, it is safer to split the
> initialization, and freeing parts into two functions:
> 
> In deferred_init_memmap()
> 1574         for_each_free_mem_range(i, nid, MEMBLOCK_NONE, &spa, &epa, NULL) {
> 1575                 spfn = max_t(unsigned long, first_init_pfn, PFN_UP(spa));
> 1576                 epfn = min_t(unsigned long, zone_end_pfn(zone),
> PFN_DOWN(epa));
> 1577                 nr_pages += deferred_init_range(nid, zid, spfn, epfn);
> 1578         }
> 
> Replace with two loops:
> First loop, calls a function that initializes the given range, the 2nd
> loop calls a function that frees it. This way we won't get a potential
> problem where buddy page is computed from the next range that has not
> yet been initialized. And it is also going to be easier to multithread
> later: multi-thread the first loop, wait for it to finish,
> multi-thread the 2nd loop wait for it to finish.

OK, but let's do that as a separate patch. What you have here is good
for now IMHO. My ack applies. Thanks!
-- 
Michal Hocko
SUSE Labs