4.1: 9a0e609e3fd ("dm: only run the queue on completion if ....") causes significant overhead in osq_lock on ext4/multipath

[Christian Borntraeger <borntraeger@de.ibm.com>]

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