On Wed, 2013-06-19 at 15:16 +0200, Ingo Molnar wrote:
> > vmstat for mutex implementation:
> > procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
> > r b swpd free buff cache si so bi bo in cs us sy id wa st
> > 38 0 0 130957920 47860 199956 0 0 0 56 236342 476975 14 72 14 0 0
> > 41 0 0 130938560 47860 219900 0 0 0 0 236816 479676 14 72 14 0 0
> >
> > vmstat for rw-sem implementation (3.10-rc4)
> > procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
> > r b swpd free buff cache si so bi bo in cs us sy id wa st
> > 40 0 0 130933984 43232 202584 0 0 0 0 321817 690741 13 71 16 0 0
> > 39 0 0 130913904 43232 224812 0 0 0 0 322193 692949 13 71 16 0 0
>
> It appears the main difference is that the rwsem variant context-switches
> about 36% more than the mutex version, right?
>
> I'm wondering how that's possible - the lock is mostly write-locked,
> correct? So the lock-stealing from Davidlohr Bueso and Michel Lespinasse
> ought to have brought roughly the same lock-stealing behavior as mutexes
> do, right?
>
> So the next analytical step would be to figure out why rwsem lock-stealing
> is not behaving in an equivalent fashion on this workload. Do readers come
> in frequently enough to disrupt write-lock-stealing perhaps?
>
> Context-switch call-graph profiling might shed some light on where the
> extra context switches come from...
>
> Something like:
>
> perf record -g -e sched:sched_switch --filter 'prev_state != 0' -a sleep 1
>
> or a variant thereof might do the trick.
>
Ingo,
It appears that we are having much more down write failure causing a process to
block vs going to the slow path for the mutex case.
Here's the profile data from
perf record -g -e sched:sched_switch --filter 'prev_state != 0' -a sleep 1
3.10-rc4 (mutex implementation context switch profile)
- 59.51% exim [kernel.kallsyms] [k] perf_trace_sched_switch
- perf_trace_sched_switch
- __schedule
- 99.98% schedule
+ 33.07% schedule_timeout
+ 23.84% pipe_wait
+ 20.24% do_wait
+ 12.37% do_exit
+ 5.34% sigsuspend
- 3.40% schedule_preempt_disabled
__mutex_lock_common.clone.8
__mutex_lock_slowpath
- mutex_lock <---------low rate mutex blocking
+ 65.71% lock_anon_vma_root.clone.24
+ 19.03% anon_vma_lock.clone.21
+ 7.14% dup_mm
+ 5.36% unlink_file_vma
+ 1.71% ima_file_check
+ 0.64% generic_file_aio_write
- 1.07% rwsem_down_write_failed
call_rwsem_down_write_failed
exit_shm
do_exit
do_group_exit
SyS_exit_group
system_call_fastpath
- 27.61% smtpbm [kernel.kallsyms] [k] perf_trace_sched_switch
- perf_trace_sched_switch
- __schedule
- schedule
- schedule_timeout
+ 100.00% sk_wait_data
+ 0.46% swapper [kernel.kallsyms] [k] perf_trace_sched_switch
----------------------
3.10-rc4 implementation (rw-sem context switch profile)
81.91% exim [kernel.kallsyms] [k] perf_trace_sched_switch
- perf_trace_sched_switch
- __schedule
- 99.99% schedule
- 65.26% rwsem_down_write_failed <------High write lock blocking
- call_rwsem_down_write_failed
- 79.36% lock_anon_vma_root.clone.27
+ 52.64% unlink_anon_vmas
+ 47.36% anon_vma_clone
+ 12.16% anon_vma_fork
+ 8.00% anon_vma_free
+ 11.96% schedule_timeout
+ 7.66% do_exit
+ 7.61% do_wait
+ 5.49% pipe_wait
+ 1.82% sigsuspend
13.55% smtpbm [kernel.kallsyms] [k] perf_trace_sched_switch
- perf_trace_sched_switch
- __schedule
- schedule
- schedule_timeout
0.11% rcu_sched [kernel.kallsyms] [k] perf_trace_sched_switch
Thanks.
Tim
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