On Wed, Dec 14, 2016 at 10:54:25AM +0100, Michal Hocko wrote:
> On Tue 13-12-16 07:14:08, Paul E. McKenney wrote:
> > Just FYI for the moment...
> >
> > So even with the slowed-down checking, making cond_resched() do what
> > cond_resched_rcu_qs() does results in a smallish but quite measurable
> > degradation according to 0day.
>
> So if I understand those results properly, the reason seems to be the
> increased involuntary context switches, right? Or am I misreading the
> data?
> I am looking at your "sched,rcu: Make cond_resched() provide RCU
> quiescent state" in linux-next and I am wondering whether rcu_all_qs has
> to be called unconditionally and not only when should_resched failed few
> times? I guess you have discussed that with Peter already but do not
> remember the outcome.
My first thought is to wait for the grace period to age further before
checking, the idea being to avoid increasing cond_resched() overhead
any further. But if that doesn't work, then yes, I may have to look at
adding more checks to cond_resched().
> Thanks for letting my know!
>
> > I will try some things to reduce the
> > impact, but it is quite possible that we will need to live with both
> > interfaces.
>
> Thanks a lot for your time!
Here is hoping for an eventual solution. ;-)
Thanx, Paul
> > ----- Forwarded message from kernel test robot <ying.huang@xxxxxxxxxxxxxxx> -----
> >
> > Date: Mon, 12 Dec 2016 13:52:28 +0800
> > From: kernel test robot <ying.huang@xxxxxxxxxxxxxxx>
> > TO: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> > Cc: lkp@xxxxxx
> > Subject: [lkp-developer] [sched,rcu] cf7a2dca60: [No primary change] +186%
> > will-it-scale.time.involuntary_context_switches
> >
> > Greeting,
> >
> > There is no primary kpi change in this test, below is the data collected through multiple monitors running background just for your information.
> >
> >
> > commit: cf7a2dca6056544bb04a8f819fbbdb415bdb2933 ("sched,rcu: Make cond_resched() provide RCU quiescent state")
> > https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev.2016.12.05c
> >
> > in testcase: will-it-scale
> > on test machine: 32 threads Intel(R) Xeon(R) CPU E5-2680 0 @ 2.70GHz with 64G memory
> > with following parameters:
> >
> > test: unlink2
> > cpufreq_governor: performance
> >
> > test-description: Will It Scale takes a testcase and runs it from 1 through to n parallel copies to see if the testcase will scale. It builds both a process and threads based test in order to see any differences between the two.
> > test-url: https://github.com/antonblanchard/will-it-scale
> >
> >
> >
> > Details are as below:
> > -------------------------------------------------------------------------------------------------->
> >
> >
> > To reproduce:
> >
> > git clone git://git.kernel.org/pub/scm/linux/kernel/git/wfg/lkp-tests.git
> > cd lkp-tests
> > bin/lkp install job.yaml # job file is attached in this email
> > bin/lkp run job.yaml
> >
> > testcase/path_params/tbox_group/run: will-it-scale/unlink2-performance/lkp-sb03
> >
> > 15705d6709cb6ba6 cf7a2dca6056544bb04a8f819f
> > ---------------- --------------------------
> > %stddev change %stddev
> > \ | \
> > 116286 114432 will-it-scale.per_process_ops
> > 20902 ± 5% 186% 59731 ± 5% will-it-scale.time.involuntary_context_switches
> > 2694 ± 8% 61% 4344 vmstat.system.cs
> > 10903 ± 99% -1e+04 148 ± 5% latency_stats.max.wait_on_page_bit.__migration_entry_wait.migration_entry_wait.do_swap_page.handle_mm_fault.__do_page_fault.do_page_fault.page_fault
> > 3583 ± 38% 1e+04 14010 ± 51% latency_stats.sum.ep_poll.SyS_epoll_wait.entry_SYSCALL_64_fastpath
> > 4143 ± 24% 1e+04 14549 ± 51% latency_stats.sum.ep_poll.SyS_epoll_wait.do_syscall_64.return_from_SYSCALL_64
> > 271108 ± 71% -2e+05 66364 ± 32% latency_stats.sum.wait_on_page_bit.__migration_entry_wait.migration_entry_wait.do_swap_page.handle_mm_fault.__do_page_fault.do_page_fault.page_fault
> > 834637 ± 8% 62% 1351381 perf-stat.context-switches
> > 16449 ± 3% 54% 25349 ± 3% perf-stat.cpu-migrations
> > 25.94 35% 35.02 perf-stat.node-store-miss-rate%
> > 2.534e+09 32% 3.335e+09 perf-stat.node-store-misses
> > 1.002e+12 4% 1.043e+12 perf-stat.dTLB-stores
> > 50923913 3% 52692115 perf-stat.iTLB-loads
> > 1.696e+12 1.745e+12 perf-stat.dTLB-loads
> > 1.258e+12 1.291e+12 perf-stat.branch-instructions
> > 6.132e+12 6.274e+12 perf-stat.instructions
> > 0.37 0.38 perf-stat.ipc
> > 0.37 -3% 0.35 perf-stat.branch-miss-rate%
> > 29.83 -4% 28.66 perf-stat.cache-miss-rate%
> > 1.117e+10 -4% 1.071e+10 perf-stat.cache-misses
> > 7.232e+09 -14% 6.187e+09 perf-stat.node-stores
> --
> Michal Hocko
> SUSE Labs
>
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