On 8/3/18 2:07 AM, Michal Hocko wrote:
On Fri 27-07-18 02:10:14, Yang Shi wrote:
When running some mmap/munmap scalability tests with large memory (i.e.
300GB), the below hung task issue may happen occasionally.
INFO: task ps:14018 blocked for more than 120 seconds.
Tainted: G E 4.9.79-009.ali3000.alios7.x86_64 #1
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
message.
ps D 0 14018 1 0x00000004
ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
Call Trace:
[<ffffffff817154d0>] ? __schedule+0x250/0x730
[<ffffffff817159e6>] schedule+0x36/0x80
[<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
[<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
[<ffffffff81717db0>] down_read+0x20/0x40
[<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0
[<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
[<ffffffff81241d87>] __vfs_read+0x37/0x150
[<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
[<ffffffff81242266>] vfs_read+0x96/0x130
[<ffffffff812437b5>] SyS_read+0x55/0xc0
[<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
It is because munmap holds mmap_sem exclusively from very beginning to
all the way down to the end, and doesn't release it in the middle. When
unmapping large mapping, it may take long time (take ~18 seconds to
unmap 320GB mapping with every single page mapped on an idle machine).
Zapping pages is the most time consuming part, according to the
suggestion from Michal Hocko [1], zapping pages can be done with holding
read mmap_sem, like what MADV_DONTNEED does. Then re-acquire write
mmap_sem to cleanup vmas.
But, some part may need write mmap_sem, for example, vma splitting. So,
the design is as follows:
acquire write mmap_sem
lookup vmas (find and split vmas)
detach vmas
deal with special mappings
downgrade_write
zap pages
free page tables
release mmap_sem
The vm events with read mmap_sem may come in during page zapping, but
since vmas have been detached before, they, i.e. page fault, gup, etc,
will not be able to find valid vma, then just return SIGSEGV or -EFAULT
as expected.
If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, they are
considered as special mappings. They will be dealt with before zapping
pages with write mmap_sem held. Basically, just update vm_flags.
Well, I think it would be safer to simply fallback to the current
implementation with these mappings and deal with them on top. This would
make potential issues easier to bisect and partial reverts as well.
Do you mean just call do_munmap()? It sounds ok. Although we may waste
some cycles to repeat what has done, it sounds not too bad since those
special mappings should be not very common.
And, since they are also manipulated by unmap_single_vma() which is
called by unmap_vma() with read mmap_sem held in this case, to
prevent from updating vm_flags in read critical section, a new
parameter, called "skip_flags" is added to unmap_region(), unmap_vmas()
and unmap_single_vma(). If it is true, then just skip unmap those
special mappings. Currently, the only place which pass true to this
parameter is us.
skip parameters are usually ugly and lead to more mess later on. Can we
do without them?
We need a way to tell unmap_region() that it is called in a kind of
special context which updating vm_flags is not allowed. I didn't think
of a better way.
We could add a new API to do what unmap_region() does without updating
vm_flags, but we would have to duplicate some code.
With this approach we don't have to re-acquire mmap_sem again to clean
up vmas to avoid race window which might get the address space changed.
By with this approach you mean detaching right?
Yes, the detaching approach.
And, since the lock acquire/release cost is managed to the minimum and
almost as same as before, the optimization could be extended to any size
of mapping without incurring significant penalty to small mappings.
I guess you mean to say that lock downgrade approach doesn't lead to
regressions because the overal time mmap_sem is taken is not longer?
Yes. And, there is not lock take/retake cost since we don't release it.
For the time being, just do this in munmap syscall path. Other
vm_munmap() or do_munmap() call sites (i.e mmap, mremap, etc) remain
intact for stability reason.
You have used this argument previously and several people have asked.
I think it is just wrong. Either the concept is safe and all callers can
use it or it is not and then those subtle differences should be called
out. Your previous response was that you simply haven't tested other
paths. Well, that is not an argument, I am afraid. The whole thing
should be done at a proper layer. If there are some difficulties to
achieve that for all callers then OK just be explicit about that. I can
imagine some callers really require the exclusive look when munmap
returns for example.
Yes, the statement here sounds ambiguous. There are definitely some
difficulties to achieve that in mmap and mremap. Since they acquire
write mmap_sem at the very beginning, then do their stuff, which may
call do_munmap if overlapped address space has to be changed.
But, the optimized do_munmap would like to be called without mmap_sem
held so that we can do the optimization. So, if we want to do the
similar optimization for mmap/mremap path, I'm afraid we would have to
redesign them.
I assumes munmap itself is the main source of the latency issue.
mmap/mremap might hit the latency problem if they are trying to map or
remap a huge overlapped address space, but it should be rare. So, I
leave them untouched.
With the patches, exclusive mmap_sem hold time when munmap a 80GB
address space on a machine with 32 cores of E5-2680 @ 2.70GHz dropped to
us level from second.
munmap_test-15002 [008] 594.380138: funcgraph_entry: | vm_munmap_zap_rlock() {
munmap_test-15002 [008] 594.380146: funcgraph_entry: !2485684 us | unmap_region();
munmap_test-15002 [008] 596.865836: funcgraph_exit: !2485692 us | }
Here the excution time of unmap_region() is used to evaluate the time of
holding read mmap_sem, then the remaining time is used with holding
exclusive lock.
I will be reading through the patch and follow up on that separately.
Thanks,
Yang
