Re: [dpdk-dev] [PATCH] lib: change rte_ring dequeue to guarantee ordering before tail update

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: Jerin Jacob [mailto:jerin.jacob@caviumnetworks.com]
> Sent: Saturday, July 23, 2016 11:39 AM
> To: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> Cc: Thomas Monjalon <thomas.monjalon@6wind.com>; Juhamatti Kuusisaari <juhamatti.kuusisaari@coriant.com>; dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] lib: change rte_ring dequeue to guarantee ordering before tail update
> 
> On Sat, Jul 23, 2016 at 10:14:51AM +0000, Ananyev, Konstantin wrote:
> > Hi lads,
> >
> > > On Sat, Jul 23, 2016 at 11:02:33AM +0200, Thomas Monjalon wrote:
> > > > 2016-07-23 8:05 GMT+02:00 Jerin Jacob <jerin.jacob@caviumnetworks.com>:
> > > > > On Thu, Jul 21, 2016 at 11:26:50PM +0200, Thomas Monjalon wrote:
> > > > >> > > Consumer queue dequeuing must be guaranteed to be done
> > > > >> > > fully before the tail is updated. This is not guaranteed
> > > > >> > > with a read barrier, changed to a write barrier just before
> > > > >> > > tail update which in
> > > practice guarantees correct order of reads and writes.
> > > > >> > >
> > > > >> > > Signed-off-by: Juhamatti Kuusisaari
> > > > >> > > <juhamatti.kuusisaari@coriant.com>
> > > > >> >
> > > > >> > Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
> > > > >>
> > > > >> Applied, thanks
> > > > >
> > > > > There was ongoing discussion on this
> > > > > http://dpdk.org/ml/archives/dev/2016-July/044168.html
> > > >
> > > > Sorry Jerin, I forgot this email.
> > > > The problem is that nobody replied to your email and you did not
> > > > nack the v2 of this patch.
> >
> > It's probably my bad.
> > I acked the patch before Jerin response, and forgot to reply later.
> >
> > > >
> > > > > This change may not be required as it has the performance impact.
> > > >
> > > > We need to clearly understand what is the performance impact
> > > > (numbers and use cases) on one hand, and is there a real bug fixed
> > > > by this patch on the other hand?
> > >
> > > IHMO, there is no real bug here. rte_smb_rmb() provides the
> > > LOAD-STORE barrier to make sure tail pointer WRITE happens only after prior LOADS.
> >
> > Yep, from what I read at the link Jerin provided, indeed it seems rte_smp_rmb() is enough for the arm arch here...
> > For ppc, as I can see both rte_smp_rmb()/rte_smp_wmb() emits the same instruction.
> >
> > >
> > > Thoughts?
> >
> > Wonder how big is a performance impact?
> 
> With this change we need to wait for addtional STORES to be completed to local buffer in addtion to LOADS from ring buffers memory.

I understand that, just wonder did you see any real performance difference?
Probably with ring_perf_autotest/mempool_perf_autotest or something?
Konstantin 

> 
> > If there is a real one, I suppose we can revert the patch?
> 
> Request to revert this one as their no benifts for other architectures and indeed it creates addtional delay in waiting for STORES to complete
> in ARM.
> Lets do the correct thing by reverting it.
> 
> Jerin
> 
> 
> 
> > Konstantin
> >
> > >
> > > >
> > > > Please guys make things clear and we'll revert if needed.