From: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
To: Jerin Jacob <jerin.jacob@caviumnetworks.com>,
Thomas Monjalon <thomas.monjalon@6wind.com>
Cc: Juhamatti Kuusisaari <juhamatti.kuusisaari@coriant.com>,
"dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [dpdk-dev] [PATCH] lib: change rte_ring dequeue to guarantee ordering before tail update
Date: Sat, 23 Jul 2016 10:14:51 +0000 [thread overview]
Message-ID: <2601191342CEEE43887BDE71AB97725836B81292@irsmsx105.ger.corp.intel.com> (raw)
In-Reply-To: <20160723093621.GA18376@localhost.localdomain>
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?
If there is a real one, I suppose we can revert the patch?
Konstantin
>
> >
> > Please guys make things clear and we'll revert if needed.
next prev parent reply other threads:[~2016-07-23 10:15 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-15 4:39 Juhamatti Kuusisaari
2016-07-15 12:22 ` Ananyev, Konstantin
2016-07-21 21:26 ` Thomas Monjalon
2016-07-23 6:05 ` Jerin Jacob
2016-07-23 9:02 ` Thomas Monjalon
2016-07-23 9:36 ` Jerin Jacob
2016-07-23 10:14 ` Ananyev, Konstantin [this message]
2016-07-23 10:38 ` Jerin Jacob
2016-07-23 11:15 ` Ananyev, Konstantin
2016-07-23 11:49 ` Jerin Jacob
2016-07-23 12:32 ` Ananyev, Konstantin
2016-07-23 12:35 ` Jerin Jacob
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