Subject: Re: [PATCH] pktgen: Avoid dirtying skb->users when txq is full

From: Stephen Hemminger <[email-masked]>
Date: Wed, 30 Sep 2009 17:25:32 -0700

On Thu, 01 Oct 2009 01:03:33 +0200
Eric Dumazet <[email-masked]> wrote:

> Stephen Hemminger a écrit :
> > On Tue, 22 Sep 2009 22:49:02 -0700
> > Stephen Hemminger <[email-masked]> wrote:
> > 
> >> I thought others want to know how to get maximum speed of pktgen.
> >>
> >> 1. Run nothing else (even X11), just a command line
> >> 2. Make sure ethernet flow control is disabled
> >>    ethtool -A eth0 autoneg off rx off tx off
> >> 3. Make sure clocksource is TSC.  On my old SMP Opteron's
> >>    needed to get patch since in 2.6.30 or later, the clock guru's
> >>    decided to remove it on all non Intel machines.  Look for patch
> >>    than enables "tsc=reliable"
> >> 4. Compile Ethernet drivers in, the overhead of the indirect
> >>    function call required for modules (or cache footprint),
> >>    slows things down.
> >> 5. Increase transmit ring size to 1000
> >>    ethtool -G eth0 tx 1000
> >>
> 
> Thanks a lot Stephen.
> 
> I did some pktgen session tonight and found one contention on skb->users field
> that following patch avoids.
> 
> 
> Before patch :
> ------------------------------------------------------------------------------
>    PerfTop:    5187 irqs/sec  kernel:100.0% [100000 cycles],  (all, cpu: 0)
> ------------------------------------------------------------------------------
> 
>              samples    pcnt   kernel function
>              _______   _____   _______________
> 
>             16688.00 - 50.9% : consume_skb
>              6541.00 - 20.0% : skb_dma_unmap
>              3277.00 - 10.0% : tg3_poll
>              1968.00 -  6.0% : mwait_idle
>               651.00 -  2.0% : irq_entries_start
>               466.00 -  1.4% : _spin_lock
>               442.00 -  1.3% : mix_pool_bytes_extract
>               373.00 -  1.1% : tg3_msi
>               353.00 -  1.1% : read_tsc
>               177.00 -  0.5% : sched_clock_local
>               176.00 -  0.5% : sched_clock
>               137.00 -  0.4% : tick_nohz_stop_sched_tick
> 
> After patch:
> ------------------------------------------------------------------------------
>    PerfTop:    3530 irqs/sec  kernel:99.9% [100000 cycles],  (all, cpu: 0)
> ------------------------------------------------------------------------------
> 
>              samples    pcnt   kernel function
>              _______   _____   _______________
> 
>             17127.00 - 34.0% : tg3_poll
>             12691.00 - 25.2% : consume_skb
>              5299.00 - 10.5% : skb_dma_unmap
>              4179.00 -  8.3% : mwait_idle
>              1583.00 -  3.1% : irq_entries_start
>              1288.00 -  2.6% : mix_pool_bytes_extract
>              1239.00 -  2.5% : tg3_msi
>              1062.00 -  2.1% : read_tsc
>               583.00 -  1.2% : _spin_lock
>               432.00 -  0.9% : sched_clock
>               416.00 -  0.8% : sched_clock_local
>               360.00 -  0.7% : tick_nohz_stop_sched_tick
>               329.00 -  0.7% : ktime_get
>               263.00 -  0.5% : _spin_lock_irqsave
> 
> I believe we could go further, batching the atomic_inc(&skb->users) we do all the
> time, competing with the atomic_dec() done by tx completion handler (possibly run
> on other cpu): Reserve XXXXXXX units to the skb->users, and decrement a pktgen
> local variable and refill the reserve if necessary, once in a while...

When buffer is allocated we know the number of times it will be cloned. So why
not set it there, would need to cleanup on interrupt but that should be possible.

Alternatively, just change skb->destructor on last packet and use a proper
completion mechanism.

> [PATCH] pktgen: Avoid dirtying skb->users when txq is full
> 
> We can avoid two atomic ops on skb->users if packet is not going to be
> sent to the device (because hardware txqueue is full)
> 
> Signed-off-by: Eric Dumazet <[email-masked]>
> ---
> 
> diff --git a/net/core/pktgen.c b/net/core/pktgen.c
> index 4d11c28..6a9ab28 100644
> --- a/net/core/pktgen.c
> +++ b/net/core/pktgen.c
> @@ -3439,12 +3439,14 @@ static void pktgen_xmit(struct pktgen_dev *pkt_dev)
>  	txq = netdev_get_tx_queue(odev, queue_map);
>  
>  	__netif_tx_lock_bh(txq);
> -	atomic_inc(&(pkt_dev->skb->users));
>  
> -	if (unlikely(netif_tx_queue_stopped(txq) || netif_tx_queue_frozen(txq)))
> +	if (unlikely(netif_tx_queue_stopped(txq) || netif_tx_queue_frozen(txq))) {
>  		ret = NETDEV_TX_BUSY;
> -	else
> -		ret = (*xmit)(pkt_dev->skb, odev);
> +		pkt_dev->last_ok = 0;
> +		goto unlock;
> +	}
> +	atomic_inc(&(pkt_dev->skb->users));
> +	ret = (*xmit)(pkt_dev->skb, odev);
>  
>  	switch (ret) {
>  	case NETDEV_TX_OK:
> @@ -3466,6 +3468,7 @@ static void pktgen_xmit(struct pktgen_dev *pkt_dev)
>  		atomic_dec(&(pkt_dev->skb->users));
>  		pkt_dev->last_ok = 0;
>  	}
> +unlock:
>  	__netif_tx_unlock_bh(txq);
>  
>  	/* If pkt_dev->count is zero, then run forever */

Acked-by: Stephen Hemminger <[email-masked]>