On Wed, 2015-01-07 at 23:45 -0800, Davidlohr Bueso wrote:
> On Thu, 2015-01-08 at 10:27 +0800, Huang Ying wrote:
> Cc'ing Peter.
Err, resending with the complete msg.
> > FYI, we noticed the below changes on
> >
> > commit c8c06efa8b552608493b7066c234cfa82c47fcea ("mm: convert i_mmap_mutex
> > to rwsem")
>
> Same exact everything, except for the lock type. No sharing going on.
>
> > testbox/testcase/testparams: lituya/unixbench/performance-execl
> >
> > 83cde9e8ba95d180 c8c06efa8b552608493b7066c2
> > ---------------- --------------------------
> > %stddev %change %stddev
> > \ | \
> > 721721 ± 1% +303.6% 2913110 ± 3%
> > unixbench.time.voluntary_context_switches
> > 11767 ± 0% -7.6% 10867 ± 1% unixbench.score
>
> And this workload appears to be from execl, right? Make sense with some
> of those numbers!!
>
> > 2.323e+08 ± 0% -7.2% 2.157e+08 ± 1%
> > unixbench.time.minor_page_faults
> > 207 ± 0% -7.0% 192 ± 1% unixbench.time.user_time
> > 4923450 ± 0% -5.7% 4641672 ± 0%
> > unixbench.time.involuntary_context_switches
> > 584 ± 0% -5.2% 554 ± 0%
> > unixbench.time.percent_of_cpu_this_job_got
> > 948 ± 0% -4.9% 902 ± 0% unixbench.time.system_time
> > 0 ± 0% +Inf% 672942 ± 2%
> > latency_stats.hits.call_rwsem_down_write_failed.vma_adjust.__split_vma.split_vma.mprotect_fixup.SyS_mprotect.system_call_fastpath
>
> What does this "hits" thing mean exactly? Since I assume both before and
> after runs have the same level of concurrency when pounding on mmap
> operations, I doubt it means that its the amount of calls into the
> slowpath... in addition the lock is obviously contended so we can forget
> about anything in the fastpath.
>
> So this is a call_rwsem_down_write_failed() vs __mutex_lock_common()
> issue.
It's late, but for some initial thoughts I believe this comes down to
differences in how mutexes and rwsems deal with ultimately blocking (and
based on the nasty sched_debug numbers reported by Huang). We now do in
call_rwsem_down_write_failed:
/* wait to be given the lock */
while (true) {
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
if (!waiter.task)
break;
schedule();
}
In which that waiter.task check forces us to need the barrier in
set_task_state() -- while in mutexes we get away with it. We could
disable preemption during this whole slowpath step and stabilize things,
just like we do with mutexes.
Thanks,
Davidlohr
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