netdev - Re: [RFC] use smp_load_acquire()/smp_store

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Date:	Wed, 29 Oct 2014 12:57:00 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Cc:	Alexander Duyck <alexander.h.duyck@...hat.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] use smp_load_acquire()/smp_store_release()

On Wed, 2014-10-29 at 12:27 -0700, Jeff Kirsher wrote:
> On Wed, 2014-10-29 at 09:16 -0700, Alexander Duyck wrote:
> > On 10/29/2014 07:49 AM, Eric Dumazet wrote:
> > > Hi Alexander
> > >
> > > The memory barriers added in commit
> > > b37c0fbe3f6dfba1f8ad2aed47fb40578a254635
> > > ("net: Add memory barriers to prevent possible race in byte queue
> > > limits")
> > >
> > > have heavy cost.
> > >
> > > It seems we could use smp_load_acquire() and smp_store_release()
> > > instead ?
> > >
> > > I'll post a patch later today. I would be interested if someone was able
> > > to test it, as your commit apparently was tested and known to fix a
> > > reproducible race.
> > >
> > > Thanks !
> 
> Eric- just CC me on the patch you post and I will see what I can do
> about getting validation eyes on it.

Thanks guys, will do, and will CC Paul as well.

Alexander, here is the following profile showing the cost of the
'mfence', in a typical rpc workload (a lot of IRQ are generated for TX
completions, because RPC tend to send small packets)

  0.11 │       je     33a
       │       mov    -0x3c(%rbp),%esi
  0.06 │       lea    0xc0(%rbx),%rdi
  0.06 │       callq  dql_completed
  0.06 │       mfence
 38.68 │       mov    0xc4(%rbx),%edx
  1.83 │       mov    0xc0(%rbx),%eax
       │       cmp    %eax,%edx
  0.22 │       js     333
  0.11 │       lock   btrl $0x1,0x98(%rbx)



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