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Date:   Wed, 20 May 2020 18:29:32 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Michal Hocko <mhocko@...nel.org>, Pavel Machek <pavel@....cz>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Robert Elliott <elliott@....com>,
        Shile Zhang <shile.zhang@...ux.alibaba.com>,
        Steven Sistare <steven.sistare@...cle.com>,
        Tejun Heo <tj@...nel.org>, Zi Yan <ziy@...dia.com>,
        linux-crypto@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-s390@...r.kernel.org,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v2 5/7] mm: parallelize deferred_init_memmap()

On Wed, May 20, 2020 at 11:27 AM Daniel Jordan
<daniel.m.jordan@...cle.com> wrote:
>
> Deferred struct page init is a significant bottleneck in kernel boot.
> Optimizing it maximizes availability for large-memory systems and allows
> spinning up short-lived VMs as needed without having to leave them
> running.  It also benefits bare metal machines hosting VMs that are
> sensitive to downtime.  In projects such as VMM Fast Restart[1], where
> guest state is preserved across kexec reboot, it helps prevent
> application and network timeouts in the guests.
>
> Multithread to take full advantage of system memory bandwidth.
>
> The maximum number of threads is capped at the number of CPUs on the
> node because speedups always improve with additional threads on every
> system tested, and at this phase of boot, the system is otherwise idle
> and waiting on page init to finish.
>
> Helper threads operate on section-aligned ranges to both avoid false
> sharing when setting the pageblock's migrate type and to avoid accessing
> uninitialized buddy pages, though max order alignment is enough for the
> latter.
>
> The minimum chunk size is also a section.  There was benefit to using
> multiple threads even on relatively small memory (1G) systems, and this
> is the smallest size that the alignment allows.
>
> The time (milliseconds) is the slowest node to initialize since boot
> blocks until all nodes finish.  intel_pstate is loaded in active mode
> without hwp and with turbo enabled, and intel_idle is active as well.
>
>     Intel(R) Xeon(R) Platinum 8167M CPU @ 2.00GHz (Skylake, bare metal)
>       2 nodes * 26 cores * 2 threads = 104 CPUs
>       384G/node = 768G memory
>
>                    kernel boot                 deferred init
>                    ------------------------    ------------------------
>     node% (thr)    speedup  time_ms (stdev)    speedup  time_ms (stdev)
>           (  0)         --   4078.0 (  9.0)         --   1779.0 (  8.7)
>        2% (  1)       1.4%   4021.3 (  2.9)       3.4%   1717.7 (  7.8)
>       12% (  6)      35.1%   2644.7 ( 35.3)      80.8%    341.0 ( 35.5)
>       25% ( 13)      38.7%   2498.0 ( 34.2)      89.1%    193.3 ( 32.3)
>       37% ( 19)      39.1%   2482.0 ( 25.2)      90.1%    175.3 ( 31.7)
>       50% ( 26)      38.8%   2495.0 (  8.7)      89.1%    193.7 (  3.5)
>       75% ( 39)      39.2%   2478.0 ( 21.0)      90.3%    172.7 ( 26.7)
>      100% ( 52)      40.0%   2448.0 (  2.0)      91.9%    143.3 (  1.5)
>
>     Intel(R) Xeon(R) CPU E5-2699C v4 @ 2.20GHz (Broadwell, bare metal)
>       1 node * 16 cores * 2 threads = 32 CPUs
>       192G/node = 192G memory
>
>                    kernel boot                 deferred init
>                    ------------------------    ------------------------
>     node% (thr)    speedup  time_ms (stdev)    speedup  time_ms (stdev)
>           (  0)         --   1996.0 ( 18.0)         --   1104.3 (  6.7)
>        3% (  1)       1.4%   1968.0 (  3.0)       2.7%   1074.7 (  9.0)
>       12% (  4)      40.1%   1196.0 ( 22.7)      72.4%    305.3 ( 16.8)
>       25% (  8)      47.4%   1049.3 ( 17.2)      84.2%    174.0 ( 10.6)
>       37% ( 12)      48.3%   1032.0 ( 14.9)      86.8%    145.3 (  2.5)
>       50% ( 16)      48.9%   1020.3 (  2.5)      88.0%    133.0 (  1.7)
>       75% ( 24)      49.1%   1016.3 (  8.1)      88.4%    128.0 (  1.7)
>      100% ( 32)      49.4%   1009.0 (  8.5)      88.6%    126.3 (  0.6)
>
>     Intel(R) Xeon(R) CPU E5-2699 v3 @ 2.30GHz (Haswell, bare metal)
>       2 nodes * 18 cores * 2 threads = 72 CPUs
>       128G/node = 256G memory
>
>                    kernel boot                 deferred init
>                    ------------------------    ------------------------
>     node% (thr)    speedup  time_ms (stdev)    speedup  time_ms (stdev)
>           (  0)         --   1682.7 (  6.7)         --    630.0 (  4.6)
>        3% (  1)       0.4%   1676.0 (  2.0)       0.7%    625.3 (  3.2)
>       12% (  4)      25.8%   1249.0 (  1.0)      68.2%    200.3 (  1.2)
>       25% (  9)      30.0%   1178.0 (  5.2)      79.7%    128.0 (  3.5)
>       37% ( 13)      30.6%   1167.7 (  3.1)      81.3%    117.7 (  1.2)
>       50% ( 18)      30.6%   1167.3 (  2.3)      81.4%    117.0 (  1.0)
>       75% ( 27)      31.0%   1161.3 (  4.6)      82.5%    110.0 (  6.9)
>      100% ( 36)      32.1%   1142.0 (  3.6)      85.7%     90.0 (  1.0)
>
>     AMD EPYC 7551 32-Core Processor (Zen, kvm guest)
>       1 node * 8 cores * 2 threads = 16 CPUs
>       64G/node = 64G memory
>
>                    kernel boot                 deferred init
>                    ------------------------    ------------------------
>     node% (thr)    speedup  time_ms (stdev)    speedup  time_ms (stdev)
>           (  0)         --   1003.7 ( 16.6)         --    243.3 (  8.1)
>        6% (  1)       1.4%    990.0 (  4.6)       1.2%    240.3 (  1.5)
>       12% (  2)      11.4%    889.3 ( 16.7)      44.5%    135.0 (  3.0)
>       25% (  4)      16.8%    835.3 (  9.0)      65.8%     83.3 (  2.5)
>       37% (  6)      18.6%    816.7 ( 17.6)      70.4%     72.0 (  1.0)
>       50% (  8)      18.2%    821.0 (  5.0)      70.7%     71.3 (  1.2)
>       75% ( 12)      19.0%    813.3 (  5.0)      71.8%     68.7 (  2.1)
>      100% ( 16)      19.8%    805.3 ( 10.8)      76.4%     57.3 ( 15.9)
>
> Server-oriented distros that enable deferred page init sometimes run in
> small VMs, and they still benefit even though the fraction of boot time
> saved is smaller:
>
>     AMD EPYC 7551 32-Core Processor (Zen, kvm guest)
>       1 node * 2 cores * 2 threads = 4 CPUs
>       16G/node = 16G memory
>
>                    kernel boot                 deferred init
>                    ------------------------    ------------------------
>     node% (thr)    speedup  time_ms (stdev)    speedup  time_ms (stdev)
>           (  0)         --    722.3 (  9.5)         --     50.7 (  0.6)
>       25% (  1)      -3.3%    746.3 (  4.7)      -2.0%     51.7 (  1.2)
>       50% (  2)       0.2%    721.0 ( 11.3)      29.6%     35.7 (  4.9)
>       75% (  3)      -0.3%    724.3 ( 11.2)      48.7%     26.0 (  0.0)
>      100% (  4)       3.0%    700.3 ( 13.6)      55.9%     22.3 (  0.6)
>
>     Intel(R) Xeon(R) CPU E5-2699 v3 @ 2.30GHz (Haswell, kvm guest)
>       1 node * 2 cores * 2 threads = 4 CPUs
>       14G/node = 14G memory
>
>                    kernel boot                 deferred init
>                    ------------------------    ------------------------
>     node% (thr)    speedup  time_ms (stdev)    speedup  time_ms (stdev)
>           (  0)         --    673.0 (  6.9)         --     57.0 (  1.0)
>       25% (  1)      -0.6%    677.3 ( 19.8)       1.8%     56.0 (  1.0)
>       50% (  2)       3.4%    650.0 (  3.6)      36.8%     36.0 (  5.2)
>       75% (  3)       4.2%    644.7 (  7.6)      56.1%     25.0 (  1.0)
>      100% (  4)       5.3%    637.0 (  5.6)      63.2%     21.0 (  0.0)
>
> On Josh's 96-CPU and 192G memory system:
>
>     Without this patch series:
>     [    0.487132] node 0 initialised, 23398907 pages in 292ms
>     [    0.499132] node 1 initialised, 24189223 pages in 304ms
>     ...
>     [    0.629376] Run /sbin/init as init process
>
>     With this patch series:
>     [    0.227868] node 0 initialised, 23398907 pages in 28ms
>     [    0.230019] node 1 initialised, 24189223 pages in 28ms
>     ...
>     [    0.361069] Run /sbin/init as init process
>
> [1] https://static.sched.com/hosted_files/kvmforum2019/66/VMM-fast-restart_kvmforum2019.pdf
>
> Signed-off-by: Daniel Jordan <daniel.m.jordan@...cle.com>
> ---
>  mm/Kconfig      |  6 ++---
>  mm/page_alloc.c | 60 ++++++++++++++++++++++++++++++++++++++++++++-----
>  2 files changed, 58 insertions(+), 8 deletions(-)
>
> diff --git a/mm/Kconfig b/mm/Kconfig
> index c1acc34c1c358..04c1da3f9f44c 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -750,13 +750,13 @@ config DEFERRED_STRUCT_PAGE_INIT
>         depends on SPARSEMEM
>         depends on !NEED_PER_CPU_KM
>         depends on 64BIT
> +       select PADATA
>         help
>           Ordinarily all struct pages are initialised during early boot in a
>           single thread. On very large machines this can take a considerable
>           amount of time. If this option is set, large machines will bring up
> -         a subset of memmap at boot and then initialise the rest in parallel
> -         by starting one-off "pgdatinitX" kernel thread for each node X. This
> -         has a potential performance impact on processes running early in the
> +         a subset of memmap at boot and then initialise the rest in parallel.
> +         This has a potential performance impact on tasks running early in the
>           lifetime of the system until these kthreads finish the
>           initialisation.
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d0c0d9364aa6d..9cb780e8dec78 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -68,6 +68,7 @@
>  #include <linux/lockdep.h>
>  #include <linux/nmi.h>
>  #include <linux/psi.h>
> +#include <linux/padata.h>
>
>  #include <asm/sections.h>
>  #include <asm/tlbflush.h>
> @@ -1814,16 +1815,44 @@ deferred_init_maxorder(u64 *i, struct zone *zone, unsigned long *start_pfn,
>         return nr_pages;
>  }
>
> +struct definit_args {
> +       struct zone *zone;
> +       atomic_long_t nr_pages;
> +};
> +
> +static void __init
> +deferred_init_memmap_chunk(unsigned long start_pfn, unsigned long end_pfn,
> +                          void *arg)
> +{
> +       unsigned long spfn, epfn, nr_pages = 0;
> +       struct definit_args *args = arg;
> +       struct zone *zone = args->zone;
> +       u64 i;
> +
> +       deferred_init_mem_pfn_range_in_zone(&i, zone, &spfn, &epfn, start_pfn);
> +
> +       /*
> +        * Initialize and free pages in MAX_ORDER sized increments so that we
> +        * can avoid introducing any issues with the buddy allocator.
> +        */
> +       while (spfn < end_pfn) {
> +               nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
> +               cond_resched();
> +       }
> +
> +       atomic_long_add(nr_pages, &args->nr_pages);
> +}
> +

Personally I would get rid of nr_pages entirely. It isn't worth the
cache thrash to have this atomic variable bouncing around. You could
probably just have this function return void since all nr_pages is
used for is a pr_info  statement at the end of the initialization
which will be completely useless now anyway since we really have the
threads running in parallel anyway.

We only really need the nr_pages logic in deferred_grow_zone in order
to track if we have freed enough pages to allow us to go back to what
we were doing.

>  /* Initialise remaining memory on a node */
>  static int __init deferred_init_memmap(void *data)
>  {
>         pg_data_t *pgdat = data;
>         const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
>         unsigned long spfn = 0, epfn = 0, nr_pages = 0;
> -       unsigned long first_init_pfn, flags;
> +       unsigned long first_init_pfn, flags, epfn_align;
>         unsigned long start = jiffies;
>         struct zone *zone;
> -       int zid;
> +       int zid, max_threads;
>         u64 i;
>
>         /* Bind memory initialisation thread to a local node if possible */
> @@ -1863,11 +1892,32 @@ static int __init deferred_init_memmap(void *data)
>                 goto zone_empty;
>
>         /*
> -        * Initialize and free pages in MAX_ORDER sized increments so
> -        * that we can avoid introducing any issues with the buddy
> -        * allocator.
> +        * More CPUs always led to greater speedups on tested systems, up to
> +        * all the nodes' CPUs.  Use all since the system is otherwise idle now.
>          */
> +       max_threads = max(cpumask_weight(cpumask), 1u);
> +
>         while (spfn < epfn) {
> +               epfn_align = ALIGN_DOWN(epfn, PAGES_PER_SECTION);
> +
> +               if (IS_ALIGNED(spfn, PAGES_PER_SECTION) &&
> +                   epfn_align - spfn >= PAGES_PER_SECTION) {
> +                       struct definit_args arg = { zone, ATOMIC_LONG_INIT(0) };
> +                       struct padata_mt_job job = {
> +                               .thread_fn   = deferred_init_memmap_chunk,
> +                               .fn_arg      = &arg,
> +                               .start       = spfn,
> +                               .size        = epfn_align - spfn,
> +                               .align       = PAGES_PER_SECTION,
> +                               .min_chunk   = PAGES_PER_SECTION,
> +                               .max_threads = max_threads,
> +                       };
> +
> +                       padata_do_multithreaded(&job);
> +                       nr_pages += atomic_long_read(&arg.nr_pages);
> +                       spfn = epfn_align;
> +               }
> +
>                 nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
>                 cond_resched();
>         }

This doesn't look right. You are basically adding threads in addition
to calls to deferred_init_maxorder. In addition you are spawning one
job per section instead of per range. Really you should be going for
something more along the lines of:

        while (spfn < epfn) {
                unsigned long epfn_align = ALIGN(epfn,
PAGE_PER_SECTION);
                struct definit_args arg = { zone, ATOMIC_LONG_INIT(0)
};
                struct padata_mt_job job = {
                        .thread_fn   = deferred_init_memmap_chunk,
                        .fn_arg      = &arg,
                        .start       = spfn,
                        .size        = epfn_align - spfn,
                        .align       = PAGES_PER_SECTION,
                        .min_chunk   = PAGES_PER_SECTION,
                        .max_threads = max_threads,
                };

                padata_do_multithreaded(&job);

                for_each_free_mem_pfn_range_in_zone_from(i, zone,
spfn, epfn) {
                        if (epfn_align <= spfn)
                                break;
                }
        }

This should accomplish the same thing, but much more efficiently. The
only thing you really lose is the tracking of nr_pages which really
doesn't add anything anyway since the value could shift around
depending on how many times deferred_grow_zone got called anyway.

Also the spfn should already be sectioned aligned, or at least be in a
new section unrelated to the one we just scheduled, so there is no
need for the extra checks you had.

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