Re: system hung up when offlining CPUs

[Thomas Gleixner]

Yasuaki,

On Mon, 16 Oct 2017, YASUAKI ISHIMATSU wrote:

> Hi Thomas,
> 
> > Can you please apply the patch below on top of Linus tree and retest?
> >
> > Please send me the outputs I asked you to provide last time in any case
> > (success or fail).
> 
> The issue still occurs even if I applied your patch to linux 4.14.0-rc4.

Thanks for testing.

> ---
> [ ...] INFO: task setroubleshootd:4972 blocked for more than 120 seconds.
> [ ...]       Not tainted 4.14.0-rc4.thomas.with.irqdebug+ #6
> [ ...] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ ...] setroubleshootd D    0  4972      1 0x00000080
> [ ...] Call Trace:
> [ ...]  __schedule+0x28d/0x890
> [ ...]  ? release_pages+0x16f/0x3f0
> [ ...]  schedule+0x36/0x80
> [ ...]  io_schedule+0x16/0x40
> [ ...]  wait_on_page_bit+0x107/0x150
> [ ...]  ? page_cache_tree_insert+0xb0/0xb0
> [ ...]  truncate_inode_pages_range+0x3dd/0x7d0
> [ ...]  ? schedule_hrtimeout_range_clock+0xad/0x140
> [ ...]  ? remove_wait_queue+0x59/0x60
> [ ...]  ? down_write+0x12/0x40
> [ ...]  ? unmap_mapping_range+0x75/0x130
> [ ...]  truncate_pagecache+0x47/0x60
> [ ...]  truncate_setsize+0x32/0x40
> [ ...]  xfs_setattr_size+0x100/0x300 [xfs]
> [ ...]  xfs_vn_setattr_size+0x40/0x90 [xfs]
> [ ...]  xfs_vn_setattr+0x87/0xa0 [xfs]
> [ ...]  notify_change+0x266/0x440
> [ ...]  do_truncate+0x75/0xc0
> [ ...]  path_openat+0xaba/0x13b0
> [ ...]  ? mem_cgroup_commit_charge+0x31/0x130
> [ ...]  do_filp_open+0x91/0x100
> [ ...]  ? __alloc_fd+0x46/0x170
> [ ...]  do_sys_open+0x124/0x210
> [ ...]  SyS_open+0x1e/0x20
> [ ...]  do_syscall_64+0x67/0x1b0
> [ ...]  entry_SYSCALL64_slow_path+0x25/0x25

This is definitely a driver issue. The driver requests an affinity managed
interrupt. Affinity managed interrupts are different from non managed
interrupts in several ways:

Non-Managed interrupts:

 1) At setup time the default interrupt affinity is assigned to each
    interrupt. The effective affinity is usually a subset of the online
    CPUs.

 2) User space can modify the affinity of the interrupt

 3) If a CPU in the affinity mask goes offline and there are still online
    CPUs in the affinity mask then the effective affinity is moved to a
    subset of the online CPUs in the affinity mask.

    If the last CPU in the affinity mask of an interrupt goes offline then
    the hotplug code breaks the affinity and makes it affine to the online
    CPUs. The effective affinity is a subset of the new affinity setting,

Managed interrupts:

 1) At setup time the interrupts of a multiqueue device are evenly spread
    over the possible CPUs. If all CPUs in the affinity mask of a given
    interrupt are offline at request_irq() time, the interrupt stays shut
    down. If the first CPU in the affinity mask comes online later the
    interrupt is started up.

 2) User space cannot modify the affinity of the interrupt

 3) If a CPU in the affinity mask goes offline and there are still online
    CPUs in the affinity mask then the effective affinity is moved a subset
    of the online CPUs in the affinity mask. I.e. the same as with
    Non-Managed interrupts.

    If the last CPU in the affinity mask of a managed interrupt goes
    offline then the interrupt is shutdown. If the first CPU in the
    affinity mask becomes online again then the interrupt is started up
    again.

So this has consequences:

 1) The device driver has to make sure that no requests are targeted at a
    queue whose interrupt is affine to offline CPUs and therefor shut
    down. If the driver ignores that then this queue will not deliver an
    interrupt simply because that interrupt is shut down.

 2) When the last CPU in the affinity mask of a queue interrupt goes
    offline the device driver has to make sure, that all outstanding
    requests in the queue which have not yet delivered their interrupt are
    completed. This is required because when the CPU is finally offline the
    interrupt is shut down and wont deliver any more interrupts.

    If that does not happen then the not yet completed request will try to
    send the completion interrupt which obviously gets not delivered
    because it is shut down.

It's hard to tell from the debug information which of the constraints (#1
or #2 or both) has been violated by the driver (or the device hardware /
firmware) but the effect that the task which submitted the I/O operation is
hung after an offline operation points clearly into that direction.

The irq core code is doing what is expected and I have no clue about that
megasas driver/hardware so I have to punt and redirect you to the SCSI and
megasas people.

Thanks,

	tglx