netdev - Re: Strange latency spikes/TX network stalls on Sun Fire X4150(x86) and e1000e

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Date:	Wed, 30 May 2012 20:29:17 +0900
From:	Hiroaki SHIMODA <shimoda.hiroaki@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	Denys Fedoryshchenko <denys@...p.net.lb>,
	netdev@...r.kernel.org, e1000-devel@...ts.sourceforge.net,
	jeffrey.t.kirsher@...el.com, jesse.brandeburg@...el.com,
	davem@...emloft.net
Subject: Re: Strange latency spikes/TX network stalls on Sun Fire X4150(x86)
 and e1000e

On Wed, 30 May 2012 13:08:27 +0200
Eric Dumazet <eric.dumazet@...il.com> wrote:

> On Wed, 2012-05-30 at 19:43 +0900, Hiroaki SHIMODA wrote:
> 
> > While examining ping problem, below pattern is often observed.
> > 
> >                                                TIME
> >        dql_queued()         dql_completed()     |
> >       a) initial state                          |
> >                                                 |
> >       b) X bytes queued                         V
> > 
> >       c) Y bytes queued
> >                            d) X bytes completed
> >       e) Z bytes queued
> >                            f) Y bytes completed
> > 
> > a) dql->limit has already some value and there is no in-flight packet.
> > b) X bytes queued.
> > c) Y bytes queued and excess limit.
> > d) X bytes completed and dql->prev_ovlimit is set and also
> >    dql->prev_num_queued is set Y.
> > e) Z bytes queued.
> > f) Y bytes completed. inprogress and prev_inprogress are true.
> > 
> > At f), if I read the comment correctly, all_prev_completed becomes
> > true and limit should be increased. But POSDIFF() ignores
> > (A == B) case, so limit is decreased.
> 
> Which POSDIFF(), because there are many ;)

I mean,
	all_prev_completed = POSDIFF(completed, dql->prev_num_queued);

> By the way, given complexity of this I suggest you split your ideas in
> independent patches.

In this case, here is the patch what I thinking.

diff --git a/lib/dynamic_queue_limits.c b/lib/dynamic_queue_limits.c
@@ -11,12 +11,14 @@
 #include <linux/dynamic_queue_limits.h>
 
 #define POSDIFF(A, B) ((A) > (B) ? (A) - (B) : 0)
+#define #define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)
 
 /* Records completed count and recalculates the queue limit */
 void dql_completed(struct dql *dql, unsigned int count)
 {
 	unsigned int inprogress, prev_inprogress, limit;
-	unsigned int ovlimit, all_prev_completed, completed;
+	unsigned int ovlimit, completed;
+	bool all_prev_completed;
 
 	/* Can't complete more than what's in queue */
 	BUG_ON(count > dql->num_queued - dql->num_completed);
@@ -26,7 +28,7 @@ void dql_completed(struct dql *dql, unsigned int count)
 	ovlimit = POSDIFF(dql->num_queued - dql->num_completed, limit);
 	inprogress = dql->num_queued - completed;
 	prev_inprogress = dql->prev_num_queued - dql->num_completed;
-	all_prev_completed = POSDIFF(completed, dql->prev_num_queued);
+	all_prev_completed = AFTER_EQ(completed, dql->prev_num_queued);
 
 	if ((ovlimit && !inprogress) ||
 	    (dql->prev_ovlimit && all_prev_completed)) {

> Mabe we should change all POSDIFF(), not selected ones.
> 
> #define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
> 
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