Re: [RFC PATCH 0/4 v2] Define killable version for access_remote_vm() and use it in fs/proc

[Yang Shi]

On 2/26/18 5:02 PM, David Rientjes wrote:
> On Tue, 27 Feb 2018, Yang Shi wrote:
>
> > Background:
> > When running vm-scalability with large memory (> 300GB), the below hung
> > task issue happens 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
> >
> > When manipulating a large mapping, the process may hold the mmap_sem for
> > long time, so reading /proc/<pid>/cmdline may be blocked in
> > uninterruptible state for long time.
> > We already have killable version APIs for semaphore, here use down_read_killable()
> > to improve the responsiveness.
> Rather than killable, we have patches that introduce down_read_unfair()
> variants for the files you've modified (cmdline and environ) as well as
> others (maps, numa_maps, smaps).

You mean you have such functionality used by google internally?

> When another thread is holding down_read() and there are queued
> down_write()'s, down_read_unfair() allows for grabbing the rwsem without
> queueing for it.  Additionally, when another thread is holding
> down_write(), down_read_unfair() allows for queueing in front of other
> threads trying to grab it for write as well.

It sounds the __unfair variant make the caller have chance to jump the 
gun to grab the semaphore before other waiters, right? But when a 
process holds the semaphore, i.e. mmap_sem, for a long time, it still 
has to sleep in uninterruptible state, right?

But, it seems __unfair variant may not be very helpful in this usecase. 
Reading /proc might be not that important to require any special care to 
grab the semaphore before other waiters. I just hope it doesn't sleep in 
uninterruptible state for a long time. If the user is not patient enough 
due to some reason, they can have a chance to abort.

> Ingo would know more about whether a variant like that in upstream Linux
> would be acceptable.
>
> Would you be interested in unfair variants instead of only addressing
> killable?

Yes, I'm although it still looks overkilling to me for reading /proc.

Thanks,
Yang