Re: [PATCH] mm: thp: use down_read_trylock in khugepaged to avoid long block
From: Yang Shi
Date: Fri Dec 15 2017 - 15:04:32 EST
Hi Kirill & Michal,
Since both of you raised the same question about who holds the semaphore
for that long time, I just reply here to both of you.
The backtrace shows vm-scalability is running with 300G memory and it is
doing munmap as below:
[188995.241865] CPU: 15 PID: 8063 Comm: usemem Tainted: G E
4.9.65-006.ali3000.alios7.x86_64 #1
[188995.242252] Hardware name: Huawei Technologies Co., Ltd. Tecal
RH2288H V2-12L/BC11SRSG1, BIOS RMIBV368 11/01/2013
[188995.242637] task: ffff883f610a5b00 task.stack: ffffc90037280000
[188995.242838] RIP: 0010:[<ffffffff811e2319>] .c [<ffffffff811e2319>]
unmap_page_range+0x619/0x940
[188995.243231] RSP: 0018:ffffc90037283c98 EFLAGS: 00000282
[188995.243429] RAX: 00002b760ac57000 RBX: 00002b760ac56000 RCX:
0000000003eb13ca
[188995.243820] RDX: ffffea003971e420 RSI: 00002b760ac56000 RDI:
ffff8837cb832e80
[188995.244211] RBP: ffffc90037283d78 R08: ffff883ebf8fc3c0 R09:
0000000000008000
[188995.244600] R10: 00000000826b7e00 R11: 0000000000000000 R12:
ffff8821e70f72b0
[188995.244993] R13: ffffea00fac4f280 R14: ffffc90037283e00 R15:
00002b760ac57000
[188995.245390] FS: 00002b34b4861700(0000) GS:ffff883f7d3c0000(0000)
knlGS:0000000000000000
[188995.245788] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[188995.245990] CR2: 00002b7092160fed CR3: 0000000977850000 CR4:
00000000001406e0
[188995.246388] Stack:
[188995.246581] 00002b92f71edfff.c 00002b7fffffffff.c
00002b92f71ee000.c ffff8809778502b0.c
[188995.246981] 00002b763fffffff.c ffff8802e1895ec0.c
ffffc90037283d48.c ffff883f610a5b00.c
[188995.247365] ffffc90037283d70.c 00002b8000000000.c
ffffc00000000fff.c ffffea00879c3df0.c
[188995.247759] Call Trace:
[188995.247957] [<ffffffff811e26bd>] unmap_single_vma+0x7d/0xe0
[188995.248161] [<ffffffff811e2a11>] unmap_vmas+0x51/0xa0
[188995.248367] [<ffffffff811e98ed>] unmap_region+0xbd/0x130
[188995.248571] [<ffffffff8170b04c>] ?
rwsem_down_write_failed_killable+0x31c/0x3f0
[188995.248961] [<ffffffff811eb94c>] do_munmap+0x26c/0x420
[188995.249162] [<ffffffff811ebbc0>] SyS_munmap+0x50/0x70
[188995.249361] [<ffffffff8170cab7>] entry_SYSCALL_64_fastpath+0x1a/0xa9
By analyzing vmcore, khugepaged is waiting for vm-scalability process's
mmap_sem.
unmap_vmas will unmap every vma in the memory space, it sounds the test
generated huge amount of vmas.
Shall we add "cond_resched()" in unmap_vmas(), i.e for every 100 vmas?
It may improve the responsiveness a little bit for non-preempt kernel,
although it still can't release the semaphore.
Thanks,
Yang
On 12/15/17 2:27 AM, Michal Hocko wrote:
On Fri 15-12-17 03:53:23, Yang Shi wrote:
In the current design, khugepaged need acquire mmap_sem before scanning
mm, but in some corner case, khugepaged may scan the current running
process which might be modifying memory mapping, so khugepaged might
block in uninterruptible state. But, the process might hold the mmap_sem
for long time when modifying a huge memory space, then 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
I am definitely interested in what the holder of the write lock does
here for such a long time.
So, it sounds pointless to just block for waiting for the semaphore for
khugepaged, here replace down_read() to down_read_trylock() to move to
scan 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 finish the
current round full_scan.
And, it soudns the change can improve khugepaged efficiency a little
bit.
The below is the test result with 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
Signed-off-by: Yang Shi <yang.s@xxxxxxxxxxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
The patch makes sense to me
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
---
mm/khugepaged.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index ea4ff25..ecc2b68 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1674,7 +1674,12 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
spin_unlock(&khugepaged_mm_lock);
mm = mm_slot->mm;
- down_read(&mm->mmap_sem);
+ /*
+ * Not wait for semaphore to avoid long time waiting, just move
+ * to the next mm on the list.
+ */
+ if (unlikely(!down_read_trylock(&mm->mmap_sem)))
+ goto breakouterloop_mmap_sem;
if (unlikely(khugepaged_test_exit(mm)))
vma = NULL;
else
--
1.8.3.1