Messages in this thread | | | Subject | Re: [RFC PATCH 0/4 v2] Define killable version for access_remote_vm() and use it in fs/proc | From | Yang Shi <> | Date | Mon, 26 Feb 2018 17:25:52 -0800 |
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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
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