Date:	Fri, 29 Apr 2016 13:28:30 -0300
From:	marcelo.leitner@...il.com
To:	Neil Horman <nhorman@...driver.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	vyasevich@...il.com, linux-sctp@...r.kernel.org,
	David.Laight@...LAB.COM, jkbs@...hat.com
Subject: Re: [PATCH v3 0/2] sctp: delay calls to sk_data_ready() as much as
 possible

On Fri, Apr 29, 2016 at 12:10:31PM -0400, Neil Horman wrote:
> On Fri, Apr 29, 2016 at 10:47:25AM -0300, marcelo.leitner@...il.com wrote:
> > On Fri, Apr 29, 2016 at 09:36:37AM -0400, Neil Horman wrote:
> > > On Thu, Apr 28, 2016 at 05:46:59PM -0300, marcelo.leitner@...il.com wrote:
> > > > On Thu, Apr 14, 2016 at 05:19:00PM -0300, marcelo.leitner@...il.com wrote:
> > > > > On Thu, Apr 14, 2016 at 04:03:51PM -0400, Neil Horman wrote:
> > > > > > On Thu, Apr 14, 2016 at 02:59:16PM -0400, David Miller wrote:
> > > > > > > From: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> > > > > > > Date: Thu, 14 Apr 2016 14:00:49 -0300
> > > > > > > 
> > > > > > > > Em 14-04-2016 10:03, Neil Horman escreveu:
> > > > > > > >> On Wed, Apr 13, 2016 at 11:05:32PM -0400, David Miller wrote:
> > > > > > > >>> From: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> > > > > > > >>> Date: Fri,  8 Apr 2016 16:41:26 -0300
> > > > > > > >>>
> > > > > > > >>>> 1st patch is a preparation for the 2nd. The idea is to not call
> > > > > > > >>>> ->sk_data_ready() for every data chunk processed while processing
> > > > > > > >>>> packets but only once before releasing the socket.
> > > > > > > >>>>
> > > > > > > >>>> v2: patchset re-checked, small changelog fixes
> > > > > > > >>>> v3: on patch 2, make use of local vars to make it more readable
> > > > > > > >>>
> > > > > > > >>> Applied to net-next, but isn't this reduced overhead coming at the
> > > > > > > >>> expense of latency?  What if that lower latency is important to the
> > > > > > > >>> application and/or consumer?
> > > > > > > >> Thats a fair point, but I'd make the counter argument that, as it
> > > > > > > >> currently
> > > > > > > >> stands, any latency introduced (or removed), is an artifact of our
> > > > > > > >> implementation rather than a designed feature of it.  That is to say,
> > > > > > > >> we make no
> > > > > > > >> guarantees at the application level regarding how long it takes to
> > > > > > > >> signal data
> > > > > > > >> readines from the time we get data off the wire, so I would rather see
> > > > > > > >> our
> > > > > > > >> throughput raised if we can, as thats been sctp's more pressing
> > > > > > > >> achilles heel.
> > > > > > > >>
> > > > > > > >>
> > > > > > > >> Thats not to say I'd like to enable lower latency, but I'd rather have
> > > > > > > >> this now,
> > > > > > > >> and start pondering how to design that in.  Perhaps we can convert the
> > > > > > > >> pending
> > > > > > > >> flag to a counter to count the number of events we enqueue, and call
> > > > > > > >> sk_data_ready every  time we reach a sysctl defined threshold.
> > > > > > > > 
> > > > > > > > That and also that there is no chance of the application reading the
> > > > > > > > first chunks before all current ToDo's are performed by either the bh
> > > > > > > > or backlog handlers for that packet. Socket lock won't be cycled in
> > > > > > > > between chunks so the application is going to wait all the processing
> > > > > > > > one way or another.
> > > > > > > 
> > > > > > > But it takes time to signal the wakeup to the remote cpu the process
> > > > > > > was running on, schedule out the current process on that cpu (if it
> > > > > > > has in fact lost it's timeslice), and then finally look at the socket
> > > > > > > queue.
> > > > > > > 
> > > > > > > Of course this is all assuming the process was sleeping in the first
> > > > > > > place, either in recv or more likely poll.
> > > > > > > 
> > > > > > > I really think signalling early helps performance.
> > > > > > > 
> > > > > > 
> > > > > > Early, yes, often, not so much :).  Perhaps what would be adventageous would be
> > > > > > to signal at the start of a set of enqueues, rather than at the end.  That would
> > > > > > be equivalent in terms of not signaling more than needed, but would eliminate
> > > > > > the signaling on every chunk.   Perhaps what you could do Marcelo would be to
> > > > > > change the sense of the signal_ready flag to be a has_signaled flag.  e.g. call
> > > > > > sk_data_ready in ulp_event_tail like we used to, but only if the has_signaled
> > > > > > flag isn't set, then set the flag, and clear it at the end of the command
> > > > > > interpreter.
> > > > > > 
> > > > > > That would be a best of both worlds solution, as long as theres no chance of
> > > > > > race with user space reading from the socket before we were done enqueuing (i.e.
> > > > > > you have to guarantee that the socket lock stays held, which I think we do).
> > > > > 
> > > > > That is my feeling too. Will work on it. Thanks :-)
> > > > 
> > > > I did the change and tested it on real machines set all for performance.
> > > > I couldn't spot any difference between both implementations.
> > > > 
> > > > Set RSS and queue irq affinity for a cpu and taskset netperf and another
> > > > app I wrote to run on another cpu. It hits socket backlog quite often
> > > > but still do direct processing every now and then.
> > > > 
> > > > With current state, netperf, scenario above. Results of perf sched
> > > > record for the CPUs in use, reported by perf sched latency:
> > > > 
> > > >   Task                  |   Runtime ms  | Switches | Average delay ms |
> > > >   Maximum delay ms | Maximum delay at       |
> > > >   netserver:3205        |   9999.490 ms |       10 | avg:    0.003 ms |
> > > >   max:    0.004 ms | max at:  69087.753356 s
> > > > 
> > > > another run
> > > >   netserver:3483        |   9999.412 ms |       15 | avg:    0.003 ms |
> > > >   max:    0.004 ms | max at:  69194.749814 s
> > > > 
> > > > With the patch below, same test:
> > > >   netserver:2643        |  10000.110 ms |       14 | avg:    0.003 ms |
> > > >   max:    0.004 ms | max at:    172.006315 s
> > > > 
> > > > another run:
> > > >   netserver:2698        |  10000.049 ms |       15 | avg:    0.003 ms |
> > > >   max:    0.004 ms | max at:    368.061672 s
> > > > 
> > > > I'll be happy to do more tests if you have any suggestions on how/what
> > > > to test.
> > > > 
> > > > ---8<---
> > > >  
> > > I think this looks reasonable, but can you post it properly please, as a patch
> > > against the head of teh net-next tree, rather than a diff from your previous
> > > work (which wasn't comitted)
> > 
> > The idea was to not officially post it yet, more just as a reference,
> > because I can't see any gains from it. I'm reluctant just due to that,
> > no strong opinion here on one way or another.
> > 
> > If you think it's better anyway to signal it early, I'll properly repost
> > it.
> > 
> Yeah, your results seem to me to indicate that for your test at least, signaling
> early vs. late doesn't make alot of difference, but Dave I think made a point in
> principle in that allowing processes to wake up when we start enqueuing can be
> better in some situations.  So all other things being equal, I'd say go with the
> method that you have here.

Okay, I'll rebase the patch and post it properly. Thanks Neil!

  Marcelo

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