[PATCH 9/9] x86/clear_huge_page: make clear_contig_region() preemptible

Ankur Arora
2023-04-03T05:24:07Z

clear_contig_region() can be used to clear up to a huge-page (2MB/1GB)
chunk Allow preemption in the irqentry_exit path to make sure we don't
hold on to the CPU for an arbitrarily long period.

Performance: vm-scalability/case-anon-w-seq-hugetlb mmaps an anonymous
hugetlb-2mb region, and then writes sequentially to the region, demand
faulting pages on the way.

This test, with a CONFIG_VOLUNTARY config shows the effects of this
change: stime drops (~18% on Icelakex, ~5% on Milan), while the utime
goes up (~15% on Icelakex, ~13% on Milan.)

*Icelakex* mm/clear_huge_page x86/clear_huge_page change
(mem=4GB/task, tasks=128)

stime 293.02 +- .49% 239.39 +- .83% -18.30%
utime 440.11 +- .28% 508.74 +- .60% +15.59%
wall-clock 5.96 +- .33% 6.27 +-2.23% + 5.20%

*Milan* mm/clear_huge_page x86/clear_huge_page change
(mem=1GB/task, tasks=512)

stime 490.95 +- 3.55% 466.90 +- 4.79% - 4.89%
utime 276.43 +- 2.85% 311.97 +- 5.15% +12.85%
wall-clock 3.74 +- 6.41% 3.58 +- 7.82% - 4.27%

The drop in stime is due to REP; STOS being more efficient for bigger
extents. The increase in utime is due to cache effects of that change:
mm/clear_huge_page() clears page-at-a-time, while narrowing towards the
faulting page; while x86/clear_huge_page only optimizes for cache
locality in the local neighbourhood of the faulting address.

This effect on utime is visible via the increased L1-dcache-load-misses
and LLC-load* and an increased backend boundedness for perf user-stat
--all-user on Icelakex. The effect is slight but given the heavy cache
pressure generated by the test, shows up in the drop in user IPC:

- 9,455,243,414,829 instructions # 2.75 insn per cycle ( +- 14.14% ) (46.17%)
- 2,367,920,864,112 L1-dcache-loads # 1.054 G/sec ( +- 14.14% ) (69.24%)
- 42,075,182,813 L1-dcache-load-misses # 2.96% of all L1-dcache accesses ( +- 14.14% ) (69.24%)
- 20,365,688 LLC-loads # 9.064 K/sec ( +- 13.98% ) (69.24%)
- 890,382 LLC-load-misses # 7.18% of all LL-cache accesses ( +- 14.91% ) (69.24%)

+ 9,467,796,660,698 instructions # 2.37 insn per cycle ( +- 14.14% ) (46.16%)
+ 2,369,973,307,561 L1-dcache-loads # 1.027 G/sec ( +- 14.14% ) (69.24%)
+ 42,155,621,201 L1-dcache-load-misses # 2.96% of all L1-dcache accesses ( +- 14.14% ) (69.24%)
+ 22,116,300 LLC-loads # 9.588 K/sec ( +- 14.20% ) (69.24%)
+ 1,355,607 LLC-load-misses # 10.29% of all LL-cache accesses ( +- 15.49% ) (69.25%)

Given the fact that the stime improves for all loads using this path,
while the utime drop is load dependent add this change.

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
arch/x86/mm/hugetlbpage.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
index 4294b77c4f18..c8564b0552e5 100644
--- a/arch/x86/mm/hugetlbpage.c
+++ b/arch/x86/mm/hugetlbpage.c
@@ -158,7 +158,17 @@ hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
static void clear_contig_region(struct page *page, unsigned long vaddr,
unsigned int npages)
{
+ might_sleep();
+
+ /*
+ * We might be clearing a large region.
+ * Allow rescheduling.
+ */
+ allow_resched();
clear_user_pages(page_address(page), vaddr, page, npages);
+ disallow_resched();
+
+ cond_resched();
}

void clear_huge_page(struct page *page,
--
2.31.1