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?
Christian
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