netdev - Re: [patch] pktgen: bug when calling ndelay in x86 architectures

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Date:	Thu, 20 Oct 2011 15:22:59 +0200
From:	Daniel Turull <daniel.turull@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Robert Olsson <robert@...julf.net>,
	Voravit Tanyingyong <voravit@....se>,
	Jens Laas <jens.laas@...m.uu.se>
Subject: Re: [patch] pktgen: bug when calling ndelay in x86 architectures

Hi,

I tested the patch and it works well.


On 10/19/2011 12:13 PM, Eric Dumazet wrote:
> Le mercredi 19 octobre 2011 à 11:33 +0200, Daniel Turull a écrit :
>> Hi,
>> then if we want to use the spin more often.
>> maybe we can increase the constant from 100000 (0.1ms) to 1000000 (1ms)?
>> How was the current value chosen?
>>
> 
> Based on user needs ;)

I think if we increase the constant to 1ms, we will reduce the jitter if we have
a rate between 1kpps and 10 kpps, but I guess is not a big deal.

I've plot this new graph with this patch:
http://tslab.ssvl.kth.se/pktgen/img/inter_eric1.eps

> 
>> I did some measurements of the inter-arrival time between packets
>> and with bigger values the maximal is reduced in the rates between
>> 2kpps and 20kpps.
>>
> 
> ndelay()/udelay() have some inaccuracies, for 'long' values, because of
> rounding errors.
> 
> If we spin, just call ktime_now() in a loop until spin_until is
> reached...
> 
> That way you get max possible resolution, given kernel time service
> constraints.
> 
> Untested patch :
> 
> diff --git a/net/core/pktgen.c b/net/core/pktgen.c
> index 38d6577..5c7e900 100644
> --- a/net/core/pktgen.c
> +++ b/net/core/pktgen.c
> @@ -2145,9 +2145,11 @@ static void spin(struct pktgen_dev *pkt_dev, ktime_t spin_until)
>  	}
>  
>  	start_time = ktime_now();
> -	if (remaining < 100000)
> -		ndelay(remaining);	/* really small just spin */
> -	else {
> +	if (remaining < 100000) {
> +		do {
> +			end_time = ktime_now();
> +		} while (ktime_lt(end_time, spin_until));
> +	} else {
>  		/* see do_nanosleep */
>  		hrtimer_init_sleeper(&t, current);
>  		do {
> @@ -2162,8 +2164,8 @@ static void spin(struct pktgen_dev *pkt_dev, ktime_t spin_until)
>  			hrtimer_cancel(&t.timer);
>  		} while (t.task && pkt_dev->running && !signal_pending(current));
>  		__set_current_state(TASK_RUNNING);
> +		end_time = ktime_now();
>  	}
> -	end_time = ktime_now();
>  
>  	pkt_dev->idle_acc += ktime_to_ns(ktime_sub(end_time, start_time));
>  	pkt_dev->next_tx = ktime_add_ns(spin_until, pkt_dev->delay);
> 
> 

Daniel
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