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.
Yang
Actually, Alexey Dobriyan had the same comment when he reviewed my
very first patch (which changes down_read to down_read_killable at
that place).
Those 4 values might be changed by prctl_set_mm() and
prctl_set_mm_map() concurrently. They used to use down_read() to
protect the change, but it looks not good enough to protect concurrent
writing. So, Mateusz Guzik's commit
ddf1d398e517e660207e2c807f76a90df543a217 ("prctl: take mmap sem for
writing to protect against others") change it to down_write().
It seems mmap_sem can be replaced to a dedicated lock. How about
defining a rwlock in mm_struct to protect those data? I will come up
with a RFC patch for this.
However, this dedicated lock just can work around this specific case.
I believe solving mmap_sem scalability issue aimed by the patch series
is still our consensus.
Thanks,
Yang
