linux-kernel - Re: [PATCH v7 3/3] nl80211/mac80211: Report signal average

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Date:	Wed, 17 Nov 2010 17:28:06 +0900
From:	Bruno Randolf <br1@...fach.org>
To:	Jouni Malinen <j@...fi>
Cc:	linville@...driver.com, randy.dunlap@...cle.com,
	peterz@...radead.org, blp@...stanford.edu,
	linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org,
	Lars_Ericsson@...ia.com, stefanr@...6.in-berlin.de,
	kosaki.motohiro@...fujitsu.com, akpm@...ux-foundation.org,
	kevin.granade@...il.com
Subject: Re: [PATCH v7 3/3] nl80211/mac80211: Report signal average

On Tue November 16 2010 18:37:43 Jouni Malinen wrote:
> On Fri, Nov 12, 2010 at 12:00:35PM +0900, Bruno Randolf wrote:
> > Extend nl80211 to report an exponential weighted moving average (EWMA) of
> > the signal value. Since the signal value usually fluctuates between
> > different packets, an average can be more useful than the value of the
> > last packet.
> > 
> > This uses the recently added generic EWMA library function.
> 
> Isn't that generic EWMA library function making this much more CPU
> intensive than necessary? I would assume the compiler is not able to
> optimize the unsigned long division in ewma_add() when the weight value
> is "hidden" in this way through ewma_init() call. While generic library
> functions can be nice, it could be useful to have an option for using an
> inline version of ewma_add that takes in avg->weight as one of the
> arguments to allow cases like weight=8 here to be implemented using
> shift right instead of unsigned long division especially when done on
> hot path..
> 
> > @@ -1158,6 +1158,7 @@ ieee80211_rx_h_sta_process(struct ieee80211_rx_data
> > *rx)
> > 
> >  	sta->last_signal = status->signal;
> > 
> > +	ewma_add(&sta->avg_signal, -status->signal);
> 
> This one here would be done for every frame received and by using a
> function call that ends up doing a potentially expensive division.
> 
> > @@ -241,6 +241,8 @@ struct sta_info *sta_info_alloc(struct
> > ieee80211_sub_if_data *sdata, +	ewma_init(&sta->avg_signal, 1000, 8);
> 
> The divisor (weight) is set to 8 here which should allow for quite a bit
> more efficient ewma_add implementation if it were to be done in a way
> that makes it easier for the compiler to see what value is being used
> (e.g., passing this 8 to ewma_add_inline()) to allow shift right to be
> used..
> 
> 
> Or am I missing something here and we have a compiler that is actually
> able to take care of this kind of optimizations by itself in the current
> ewma library design? Or is the unsigned long division with the expected
> weight values going to be fast enough to not justify caring about it
> even on any of the embedded boards anyway?

I understand that this could be more efficient, but if it matters or not - 
honestly, I don't know. Can a more knowledgeable person than me comment on 
this?

bruno
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