netdev - Re: [PATCH] sch_red: fix red_calc_qavg_from_idle

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Date:	Wed, 30 Nov 2011 22:40:13 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>,
	Stephen Hemminger <shemminger@...tta.com>,
	David Täht <dave.taht@...il.com>
Subject: Re: [PATCH] sch_red: fix red_calc_qavg_from_idle_time

Le mercredi 30 novembre 2011 à 22:34 +0100, Eric Dumazet a écrit :
> Since commit a4a710c4a7490587 (pkt_sched: Change PSCHED_SHIFT from 10 to
> 6) it seems [G]RED are broken in red_calc_qavg_from_idle_time()
> 
> This function computes a delay in us units, but this delay is now 16
> times bigger than real delay, so the final qavg result is wrong...
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> ---
> Maybe we should use native kernel time services in red ?
> (ktime_get(), ktime_us_delta())
> 
>  include/net/pkt_sched.h |    1 +
>  include/net/red.h       |    2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h
> index fffdc60..c719d9b 100644
> --- a/include/net/pkt_sched.h
> +++ b/include/net/pkt_sched.h
> @@ -43,6 +43,7 @@ typedef long	psched_tdiff_t;
>  /* Avoid doing 64 bit divide */
>  #define PSCHED_SHIFT			6
>  #define PSCHED_TICKS2NS(x)		((s64)(x) << PSCHED_SHIFT)
> +#define PSCHED_TICKS2US(x)		(PSCHED_TICKS2NS(x) >> 10) /* approximate 1000 by 1024 */
>  #define PSCHED_NS2TICKS(x)		((x) >> PSCHED_SHIFT)
>  
>  #define PSCHED_TICKS_PER_SEC		PSCHED_NS2TICKS(NSEC_PER_SEC)
> diff --git a/include/net/red.h b/include/net/red.h
> index 3319f16..a413638 100644
> --- a/include/net/red.h
> +++ b/include/net/red.h
> @@ -175,7 +175,7 @@ static inline unsigned long red_calc_qavg_from_idle_time(struct red_parms *p)
>  	int  shift;
>  
>  	now = psched_get_time();
> -	us_idle = psched_tdiff_bounded(now, p->qidlestart, p->Scell_max);
> +	us_idle = PSCHED_TICKS2US(psched_tdiff_bounded(now, p->qidlestart, p->Scell_max));

Hmm, this is not correct the bound should be applied on the us result,
not on tick I presume (one tick == 64ns)

I'll test another patch.



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