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