Subject: Re: [PATCH net-next] net: properly flush delay-freed skbs

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 23 Nov 2016 19:11:58 +0100

On Wed, 23 Nov 2016 09:12:50 -0800
Alexander Duyck <[email-masked]> wrote:

> On Wed, Nov 23, 2016 at 8:44 AM, Eric Dumazet <[email-masked]> wrote:
> > From: Eric Dumazet <[email-masked]>
> >
> > Typical NAPI drivers use napi_consume_skb(skb) at TX completion time.
> > This put skb in a percpu special queue, napi_alloc_cache, to get bulk
> > frees.
> >
> > It turns out the queue is not flushed and hits the NAPI_SKB_CACHE_SIZE
> > limit quite often, with skbs that were queued hundreds of usec earlier.
> > I measured this can take ~6000 nsec to perform one flush.
> >
> > __kfree_skb_flush() can be called from two points right now :
> >
> > 1) From net_tx_action(), but only for skbs that were queued to
> > sd->completion_queue.
> >  
> >  -> Irrelevant for NAPI drivers in normal operation.  
> >
> > 2) From net_rx_action(), but only under high stress or if RPS/RFS has a
> > pending action.
> >
> > This patch changes net_rx_action() to perform the flush in all cases and
> > after more urgent operations happened (like kicking remote CPUS for
> > RPS/RFS).
> >
> > Signed-off-by: Eric Dumazet <[email-masked]>
> > Cc: Jesper Dangaard Brouer <[email-masked]>
> > Cc: Alexander Duyck <[email-masked]>
> > ---  
> 
> Yeah, we didn't intent the data to be sitting around that long.  The
> change looks good to me.
> 
> Acked-by: Alexander Duyck <[email-masked]>

Also looks good to me! Thanks for catching this.

Acked-by: Jesper Dangaard Brouer <[email-masked]>

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer