On Mon, Jan 20, 2014 at 08:41:00PM -0800, Paul E. McKenney wrote:
> On Mon, Jan 20, 2014 at 08:29:12PM +0800, Fengguang Wu wrote:
> > On Sun, Jan 19, 2014 at 03:11:14PM -0800, Paul E. McKenney wrote:
> > > On Sun, Jan 19, 2014 at 08:16:08PM +0800, Fengguang Wu wrote:
> > > > Hi Paul,
> > > >
> > > > Just FYI, we noticed the following changes (which looks good) on old
> > > > commit
> > > > c0f4dfd4f9 ("rcu: Make RCU_FAST_NO_HZ take advantage of numbered
> > > > callbacks")
> > > > in test case dd-write/4HDD-JBOD-cfq-btrfs-1dd:
> > > >
> > > > b11cc5 (parent) c0f4dfd4f90f1667d234d21f1
> > > > --------------- -------------------------
> > > > 213757 ~ 4% -65.4% 73929 ~ 3% softirqs.RCU
> > > > 21193 ~ 5% -36.5% 13451 ~ 4% softirqs.SCHED
> > > > 2036 ~ 4% -59.4% 825 ~ 3% vmstat.system.cs
> > > > 1304520 ~ 4% -59.2% 532451 ~ 3% perf-stat.context-switches
> > > > 95685 ~ 4% -44.0% 53598 ~ 2% perf-stat.cpu-migrations
> > >
> > > Glad it helped! IIRC, this same commit increased latencies due to
> > > synchronize_rcu() latency increasing. So this is the good side of
> > > that other not-so-good result. ;-)
> >
> > If you care it and there is a low cost way for user space to get that
> > synchronize_rcu() latency, I'd be eager to collect it in my tests. :)
>
> Would a kernel module that measured the latency be OK, or do you need
> some system call that is exposed to synchronize_rcu() latency?
Kernel module should be good enough for me. Perhaps something like
kernel/latencytop.c?
Thanks,
Fengguang
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