On 3/22/18 2:10 AM, Michal Hocko wrote:
On Wed 21-03-18 15:36:12, Yang Shi wrote:
On 3/21/18 2:23 PM, Michal Hocko wrote:
On Wed 21-03-18 10:16:41, Yang Shi wrote:
On 3/21/18 9:50 AM, Yang Shi wrote:
On 3/21/18 6:14 AM, Michal Hocko wrote:
On Wed 21-03-18 05:31:19, 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
Slightly off-topic:
Btw. this sucks as well. Do we really need to take mmap_sem here? Do any
of
arg_start = mm->arg_start;
arg_end = mm->arg_end;
env_start = mm->env_start;
env_end = mm->env_end;
change after exec or while the pid is already visible in proc? If yes
maybe we can use a dedicated lock.
BTW, this is not the only place to acquire mmap_sem in
proc_pid_cmdline_read(), it calls access_remote_vm() which need acquire
mmap_sem too, so the mmap_sem scalability issue will be hit sooner or later.
Ohh, absolutely. mmap_sem is unfortunatelly abused and it would be great
to remove that. munmap should perform much better. How to do that safely
Yes, agree. We are on the same page.
is a different question. I am not yet convinced that tearing down a vma
in batches is safe. The vast majority of time is spent on tearing down
You can try my patches. I did full LTP test and running multiple kernel
build in parallel. It survives.
Which doesn't really mean anything. Those tests are likely to not hit
corner cases where an application silently depends on the mmap locking
and unmap atomicity.
They definitely can't cover all corner cases. But, they do give us
somehow confidence that the most part works. The mmap stress tests in
LTP did discover some race conditions when I tried different approaches.
pages and that is quite easy to move out of the write lock. That would
be an improvement already and it should be risk safe. If even that is
not sufficient then using range locking should help a lot. There
shouldn't be really any other address space operations within the range
most of the time so this would be basically non-contended access.
It might depend on how the range is defined. Too big range may lead to
surprisingly more contention, but too small range may bring in too much
lock/unlock operations.
The full vma will have to be range locked. So there is nothing small or large.
It sounds not helpful to a single large vma case since just one range
lock for the vma, it sounds equal to mmap_sem.
Yang
