On Wed, Jul 08 2015 at 4:37am -0400,
Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
> not sure if this actually a mutex/s390/dm or ext4 problem, but it bisects down to a dm commit:
>
> Mike,
>
> commit 9a0e609e3fd8a95c96629b9fbde6b8c5b9a1456a ("dm: only run the queue on
> completion if congested or no requests pending") causes a significant overhead
> if multiple processes access the same file on an ext4 file system on multipath,
> with direct io.
>
> This actually appeared first with a kvm guest that has I/O on 500 virtio disks that
> are backed up by the same image file (I used this to test something else)
>
> but something like the following (without kvm)
>
> for ((d=1; d<500; d++)); do dd if=fileonmultipathext4 of=/dev/null bs=4096 iflag=direct & done
>
> keeps most CPUs on the osq_lock (optimistic spinning for mutex)
>
> # Overhead Command Shared Object Symbol
> # ........ .............. ................... ...........................................
> #
> 73.91% dd [kernel.vmlinux] [k] osq_lock
> 3.15% dd [kernel.vmlinux] [k] mutex_spin_on_owner.isra.5
> 3.03% swapper [kernel.vmlinux] [k] osq_lock
> 1.08% kdmwork-252:28 [kernel.vmlinux] [k] osq_lock
> 0.91% dd [kernel.vmlinux] [k] arch_spin_lock_wait_flags
> 0.36% dd [kernel.vmlinux] [k] kmem_cache_free
> 0.29% dd [kernel.vmlinux] [k] account_system_time
> 0.25% dd [kernel.vmlinux] [k] __blockdev_direct_IO
> 0.25% dd [kernel.vmlinux] [k] _mix_pool_bytes
> 0.22% dd [kernel.vmlinux] [k] __schedule
> 0.22% dd [kernel.vmlinux] [k] pcpu_ec_call
> 0.21% dd [kernel.vmlinux] [k] enqueue_entity
> 0.18% dd [kernel.vmlinux] [k] vtime_account_irq_enter
> 0.17% dd [dm_multipath] [k] multipath_status
> 0.17% dd [kernel.vmlinux] [k] try_to_wake_up
> 0.17% dd [kernel.vmlinux] [k] update_cfs_shares
> 0.16% dd [kernel.vmlinux] [k] blk_update_request
>
> With that patch reverted the system is much less contendent on osq_lock
>
> # Overhead Command Shared Object Symbol
> # ........ .............. ................. ...........................................
> #
> 30.22% dd [kernel.vmlinux] [k] osq_lock
> 5.57% dd [kernel.vmlinux] [k] mutex_spin_on_owner.isra.5
> 5.48% swapper [kernel.vmlinux] [k] osq_lock
> 1.61% dd [kernel.vmlinux] [k] arch_spin_lock_wait_flags
> 1.38% dd [kernel.vmlinux] [k] arch_spin_lock_wait
> 1.17% swapper [kernel.vmlinux] [k] mutex_spin_on_owner.isra.5
> 0.67% dd [kernel.vmlinux] [k] kmem_cache_free
> 0.63% dd [kernel.vmlinux] [k] try_to_wake_up
> 0.63% kdmwork-252:22 [kernel.vmlinux] [k] osq_lock
> 0.57% dd [kernel.vmlinux] [k] _mix_pool_bytes
> 0.57% dd [kernel.vmlinux] [k] account_system_time
> 0.49% dd [kernel.vmlinux] [k] pcpu_ec_call
> 0.48% dd [kernel.vmlinux] [k] vtime_account_irq_enter
> 0.44% dd [kernel.vmlinux] [k] zfcp_fsf_reqid_check
> 0.42% dd [kernel.vmlinux] [k] __blockdev_direct_IO
> 0.42% dd [kernel.vmlinux] [k] enqueue_entity
>
>
> Do you have any idea why this patch seems to affect mutex/sem hold times?
No. Only thing I can think of is that it is creating additional back
pressure on the system and IO isn't proceeding as quickly as before.
This commit has shown itself to be a problem with extensive multipath IO
fault injection tests under load (excessive IO stalls). So I intend to
revert it anyway -- this report helps cement the need for revert (I'll
cc stable@ too so it'll fix 4.1).
Mike
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