linux-kernel - Re: [RFC PATCH 2/2] ptr_ring_ll: pop/push multiple objects at once

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Date:   Wed, 16 Nov 2016 07:23:25 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     jasowang@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] ptr_ring_ll: pop/push multiple objects at once

On Tue, Nov 15, 2016 at 08:42:03PM -0800, John Fastabend wrote:
> On 16-11-14 03:06 PM, Michael S. Tsirkin wrote:
> > On Thu, Nov 10, 2016 at 08:44:32PM -0800, John Fastabend wrote:
> >> Signed-off-by: John Fastabend <john.r.fastabend@...el.com>
> > 
> > This will naturally reduce the cache line bounce
> > costs, but so will a _many API for ptr-ring,
> > doing lock-add many-unlock.
> > 
> > the number of atomics also scales better with the lock:
> > one per push instead of one per queue.
> > 
> > Also, when can qdisc use a _many operation?
> > 
> 
> On dequeue we can pull off many skbs instead of one at a time and
> then either (a) pass them down as an array to the driver (I started
> to write this on top of ixgbe and it seems like a win) or (b) pass
> them one by one down to the driver and set the xmit_more bit correctly.
> 
> The pass one by one also seems like a win because we avoid the lock
> per skb.
> 
> On enqueue qdisc side its a bit more evasive to start doing this.
> 
> 
> [...]


I see. So we could wrap __ptr_ring_consume and
implement __skb_array_consume. You can call that
in a loop under a lock. I would limit it to something
small like 16 pointers, to make sure lock contention is
not an issue.

-- 
MST