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

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Date:	Wed, 30 May 2012 16:49:02 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hiroaki SHIMODA <shimoda.hiroaki@...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, 2012-05-30 at 20:29 +0900, Hiroaki SHIMODA wrote:
> 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)) {

I am fine with this one.

Can you send official patches please ?



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