[Drbd-dev] [CASE-30] dec_ap_bio wakeup condition difference between V8.4.7 and V9.0.1

[Jaeheon Kim]

Hi,


I have one more question to the case-29.

According to the Windows and Linux DRBD kernel stack, there are a similar
pattern.

1. Stacks

1) Windows Stack dump when the file copy command was pending.

   - Please see "inc_ap_bio"

    nt!KiSwapThread+0x266
    nt!KiCommitThreadWait+0x1df
    nt!KeWaitForMultipleObjects+0x535
    drbd!schedule+0x114
    drbd!inc_ap_bio+0x49
    drbd!drbd_make_request+0x29
    drbd!DoSplitIo+0xfb
    drbd!mvolReadWriteDevice+0x2b0
    drbd!mvolWorkThread+0x114


2) Linux Stack dump when the file copy command was pending.

  - Pleae check previous post.

    [Drbd-dev] [CASE-29] After re-connect, WFBitMapS-WFBitMapT status has
sustained
                                    continuously and copy command hangs
     http://lists.linbit.com/pipermail/drbd-dev/2016-March/003328.html


  - Please see the "7) drbd pending point #1"

    [CASE-29] primary log
     http://pastebin.com/3hAWNkHn

    Mar  3 14:52:04 drbd9-01 kernel: Call Trace:
    Mar  3 14:52:04 drbd9-01 kernel: [<ffffffff816096a9>] schedule+0x29/0x70
    Mar  3 14:52:04 drbd9-01 kernel: [<ffffffffa04a03da>]
drbd_make_request+0x14a/0x360 [drbd]
    Mar  3 14:52:04 drbd9-01 kernel: [<ffffffff812aa56f>] ?
generic_make_request_checks+0x24f/0x380
    Mar  3 14:52:04 drbd9-01 kernel: [<ffffffff81098230>] ?
wake_up_bit+0x30/0x30
    Mar  3 14:52:04 drbd9-01 kernel: [<ffffffff812aa782>]
generic_make_request+0xe2/0x130
    Mar  3 14:52:04 drbd9-01 kernel: [<ffffffff812aa841>]
submit_bio+0x71/0x150
    Mar  3 14:52:04 drbd9-01 kernel: [<ffffffffa0531c55>]
ext4_io_submit+0x25/0x50 [ext4]



2. Sources

1) Ver 9.0.1

    static inline void inc_ap_bio(struct drbd_device *device, int rw)
    {
        wait_event(device->misc_wait, inc_ap_bio_cond(device, rw));
    }

    static inline void dec_ap_bio(struct drbd_device *device, int rw)
    {
         int ap_bio = atomic_dec_return(&device->ap_bio_cnt[rw]);
       .........

         if (ap_bio == 0)
                wake_up(&device->misc_wait);
    }


2) Ver 8.4.7

    static inline void inc_ap_bio(struct drbd_device *device)
    {
         wait_event(device->misc_wait, inc_ap_bio_cond(device));
    }

    static inline void dec_ap_bio(struct drbd_device *device)
    {
         int mxb = drbd_get_max_buffers(device);
         int ap_bio = atomic_dec_return(&device->ap_bio_cnt);

        .............


         /* this currently does wake_up for every dec_ap_bio!
          * maybe rather introduce some type of hysteresis?
          * e.g. (ap_bio == mxb/2 || ap_bio == 0) ? */

          if (ap_bio < mxb)
                 wake_up(&device->misc_wait);
    }



3. Questions

1) See "1. Stacks".
    I think, The Linux "drbd_make_request+0x14a/0x360 [drbd]" will probably
    mean the same thing as the Windows "drbd!inc_ap_bio+0x49".
    What do you think about it?

2) In version 8.4.7, dec_ap_bio wakes up misc_wait when condition is
"ap_bio < max".
    I mean, this version has greater tolerance than version 9.0.1.
    My question is Why do you change this condition from "ap_bio < max" to
"ap_bio == 0"  in version 9.0.1?


Thanks.
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