Re: [PATCH 0/6] ipc/sem.c: performance improvements, FIFO

[Manfred Spraul]

On 06/14/2013 09:05 PM, Mike Galbraith wrote:
> # Events: 802K cycles
> #
> # Overhead                                      Symbol
> # ........  ..........................................
> #
>      18.42%  [k] SYSC_semtimedop
>      15.39%  [k] sem_lock
>      10.26%  [k] _raw_spin_lock
>       9.00%  [k] perform_atomic_semop
>       7.89%  [k] system_call
>       7.70%  [k] ipc_obtain_object_check
>       6.95%  [k] ipcperms
>       6.62%  [k] copy_user_generic_string
>       4.16%  [.] __semop
>       2.57%  [.] worker_thread(void*)
>       2.30%  [k] copy_from_user
>       1.75%  [k] sem_unlock
>       1.25%  [k] ipc_obtain_object
~ 280 mio ops.
2.3% copy_from_user,
9% perform_atomic_semop.

> # Events: 802K cycles
> #
> # Overhead                           Symbol
> # ........  ...............................
> #
>      17.38%  [k] SYSC_semtimedop
>      13.26%  [k] system_call
>      11.31%  [k] copy_user_generic_string
>       7.62%  [.] __semop
>       7.18%  [k] _raw_spin_lock
>       5.66%  [k] ipcperms
>       5.40%  [k] sem_lock
>       4.65%  [k] perform_atomic_semop
>       4.22%  [k] ipc_obtain_object_check
>       4.08%  [.] worker_thread(void*)
>       4.06%  [k] copy_from_user
>       2.40%  [k] ipc_obtain_object
>       1.98%  [k] pid_vnr
>       1.45%  [k] wake_up_sem_queue_do
>       1.39%  [k] sys_semop
>       1.35%  [k] sys_semtimedop
>       1.30%  [k] sem_unlock
>       1.14%  [k] security_ipc_permission
~ 700 mio ops.
4% copy_from_user -> as expected a bit more
4.6% perform_atomic_semop --> less.

Thus: Could you send the oprofile output from perform_atomic_semop()?

Perhaps that gives us a hint.

My current guess:
sem_lock() somehow ends up in lock_array.
Lock_array scans all struct sem -> transfer of that cacheline from all 
cpus to the cpu that does the lock_array..
Then the next write by the "correct" cpu causes a transfer back when 
setting sem->pid.

--
    Manfred
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