The patch titled
Subject: mm: thp: use down_read_trylock() in khugepaged to avoid long block
has been removed from the -mm tree. Its filename was
mm-thp-use-down_read_trylock-in-khugepaged-to-avoid-long-block.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
From: "Yang Shi" <yang.s@xxxxxxxxxxxxxxx>
Subject: mm: thp: use down_read_trylock() in khugepaged to avoid long block
In the current design, khugepaged needs to acquire mmap_sem before
scanning an mm. But in some corner cases, khugepaged may scan a process
which is modifying its memory mapping, so khugepaged blocks in
uninterruptible state. But the process might hold the mmap_sem for a long
time when modifying a huge memory space and it may trigger the below
khugepaged hung issue:
INFO: task khugepaged:270 blocked for more than 120 seconds.
Tainted: G E 4.9.65-006.ali3000.alios7.x86_64 #1
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
khugepaged D 0 270 2 0x00000000
ffff883f3deae4c0 0000000000000000 ffff883f610596c0 ffff883f7d359440
ffff883f63818000 ffffc90019adfc78 ffffffff817079a5 d67e5aa8c1860a64
0000000000000246 ffff883f7d359440 ffffc90019adfc88 ffff883f610596c0
Call Trace:
[<ffffffff817079a5>] ? __schedule+0x235/0x6e0
[<ffffffff81707e86>] schedule+0x36/0x80
[<ffffffff8170a970>] rwsem_down_read_failed+0xf0/0x150
[<ffffffff81384998>] call_rwsem_down_read_failed+0x18/0x30
[<ffffffff8170a1c0>] down_read+0x20/0x40
[<ffffffff81226836>] khugepaged+0x476/0x11d0
[<ffffffff810c9d0e>] ? idle_balance+0x1ce/0x300
[<ffffffff810d0850>] ? prepare_to_wait_event+0x100/0x100
[<ffffffff812263c0>] ? collapse_shmem+0xbf0/0xbf0
[<ffffffff810a8d46>] kthread+0xe6/0x100
[<ffffffff810a8c60>] ? kthread_park+0x60/0x60
[<ffffffff8170cd15>] ret_from_fork+0x25/0x30
So it sounds pointless to just block khugepaged waiting for the semaphore
so replace down_read() with down_read_trylock() to move to scan the next
mm quickly instead of just blocking on the semaphore so that other
processes can get more chances to install THP. Then khugepaged can come
back to scan the skipped mm when it has finished the current round
full_scan.
And it appears that the change can improve khugepaged efficiency a little
bit.
Below is the test result when running LTP on a 24 cores 4GB memory 2 nodes
NUMA VM:
pristine w/ trylock
full_scan 197 187
pages_collapsed 21 26
thp_fault_alloc 40818 44466
thp_fault_fallback 18413 16679
thp_collapse_alloc 21 150
thp_collapse_alloc_failed 14 16
thp_file_alloc 369 369
[akpm@xxxxxxxxxxxxxxxxxxxx: coding-style fixes]
[akpm@xxxxxxxxxxxxxxxxxxxx: tweak comment]
[arnd@xxxxxxxx: avoid uninitialized variable use]
Link: http://lkml.kernel.org/r/20171215125129.2948634-1-arnd@xxxxxxxx
Link: http://lkml.kernel.org/r/1513281203-54878-1-git-send-email-yang.s@xxxxxxxxxxxxxxx
Signed-off-by: Yang Shi <yang.s@xxxxxxxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/khugepaged.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
diff -puN mm/khugepaged.c~mm-thp-use-down_read_trylock-in-khugepaged-to-avoid-long-block mm/khugepaged.c
--- a/mm/khugepaged.c~mm-thp-use-down_read_trylock-in-khugepaged-to-avoid-long-block
+++ a/mm/khugepaged.c
@@ -1673,10 +1673,14 @@ static unsigned int khugepaged_scan_mm_s
spin_unlock(&khugepaged_mm_lock);
mm = mm_slot->mm;
- down_read(&mm->mmap_sem);
- if (unlikely(khugepaged_test_exit(mm)))
- vma = NULL;
- else
+ /*
+ * Don't wait for semaphore (to avoid long wait times). Just move to
+ * the next mm on the list.
+ */
+ vma = NULL;
+ if (unlikely(!down_read_trylock(&mm->mmap_sem)))
+ goto breakouterloop_mmap_sem;
+ if (likely(!khugepaged_test_exit(mm)))
vma = find_vma(mm, khugepaged_scan.address);
progress++;
_
Patches currently in -mm which might be from yang.s@xxxxxxxxxxxxxxx are
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