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

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Date:   Wed, 23 Nov 2016 19:11:58 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        brouer@...hat.com
Subject: Re: [PATCH net-next] net: properly flush delay-freed skbs

On Wed, 23 Nov 2016 09:12:50 -0800
Alexander Duyck <alexander.duyck@...il.com> wrote:

> On Wed, Nov 23, 2016 at 8:44 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > 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 <edumazet@...gle.com>
> > Cc: Jesper Dangaard Brouer <brouer@...hat.com>
> > Cc: Alexander Duyck <alexander.h.duyck@...el.com>
> > ---  
> 
> Yeah, we didn't intent the data to be sitting around that long.  The
> change looks good to me.
> 
> Acked-by: Alexander Duyck <alexander.h.duyck@...el.com>

Also looks good to me! Thanks for catching this.

Acked-by: Jesper Dangaard Brouer <brouer@...hat.com>

-- 
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