On Wed, 2015-01-07 at 23:50 -0800, Davidlohr Bueso wrote:
> 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();
> }
heh I was actually looking at the reader code. We really do:
/* wait until we successfully acquire the lock */
set_current_state(TASK_UNINTERRUPTIBLE);
while (true) {
if (rwsem_try_write_lock(count, sem))
break;
raw_spin_unlock_irq(&sem->wait_lock);
/* Block until there are no active lockers. */
do {
schedule();
set_current_state(TASK_UNINTERRUPTIBLE);
} while ((count = sem->count) & RWSEM_ACTIVE_MASK);
raw_spin_lock_irq(&sem->wait_lock);
}
Which still has similar issues with even two barriers, I guess for both
the rwsem_try_write_lock call (less severe) and count checks. Anyway...
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