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

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Date:	Wed, 19 Oct 2011 12:13:55 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Daniel Turull <daniel.turull@...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

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 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);


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