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

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Date:   Fri, 25 Nov 2016 19:49:57 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     netdev@...r.kernel.org, brouer@...hat.com,
        alexander.h.duyck@...el.com
Subject: Re: [PATCH net-next] net: properly flush delay-freed skbs

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 23 Nov 2016 08:44:56 -0800

> 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>

Applied.