linux-kernel - Re: [PATCH tip/core/rcu 0/21] Contention reduction for v4.18

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Date:   Tue, 15 May 2018 08:21:20 +1000
From:   Nicholas Piggin <nicholas.piggin@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel.opensrc@...il.com,
        torvalds@...ux-foundation.org, npiggin@...il.com
Subject: Re: [PATCH tip/core/rcu 0/21] Contention reduction for v4.18

On Mon, 14 May 2018 09:09:07 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> On Mon, May 14, 2018 at 04:42:33PM +1000, Nicholas Piggin wrote:
> > On Sun, 22 Apr 2018 20:02:58 -0700
> > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> >   
> > > Hello!
> > > 
> > > This series reduces lock contention on the root rcu_node
> > > structure, and is also the first precursor to TBD changes to
> > > consolidate the three RCU flavors (RCU-bh, RCU-preempt, and
> > > RCU-sched) into one.  
> > 
> > Hi Paul,
> > 
> > I've been running your rcu/dev branch and haven't noticed any
> > problems yet. The irqsoff latency improvement is a little hard to
> > measure because the scheduler, but I've tried turning balancing
> > parameters right down and I'm yet to see any sign of RCU in traces
> > (down to about 100us on a 176 CPU machine), so that's great.  
> 
> Good to hear!!!

Yep, as in, various other latencies are down to 100us, and still
no sign of RCU, so RCU must be sitting somewhere below that.


> > (Not that RCU was ever the worst contributor to latency as I said,
> > just that I noticed those couple of traces where it showed up.)
> > 
> > Thanks very much for the fast response, sorry I've taken a while to
> > test.  
> 
> Would you be willing to give me a Tested-by on that series of patches?

Yes of course, for your rcu/dev series

Tested-by: Nicholas Piggin <npiggin@...il.com>

Let me know if you make any other changes you'd like me to test before
merge.

Thanks,
Nick