[PATCH v2 5/7] mm: parallelize deferred_init_memmap()

From: Daniel Jordan
Date: Wed May 20 2020 - 14:28:29 EST

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@xxxxxxxxxx>
---
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);
+}
+
/* 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();
}
--
2.26.2