netdev - Re: [PATCH net-next] sctp: support per-association stats via a new SCTP_GET_ASSOC

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Message-ID: <20121030125230.GB13450@casper.infradead.org>
Date:	Tue, 30 Oct 2012 12:52:30 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Vlad Yasevich <vyasevich@...il.com>
Cc:	Neil Horman <nhorman@...driver.com>,
	Michele Baldessari <michele@...syn.org>,
	linux-sctp@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] sctp: support per-association stats via a new
 SCTP_GET_ASSOC_STATS call

On 10/29/12 at 04:22pm, Vlad Yasevich wrote:
> On 10/29/2012 07:37 AM, Neil Horman wrote:
> >Hm, ok, looking for the maximum rto seen is definately more efficient that a
> >high polling rate on the remaddr file.  Still can't say I really like it as a
> >statistic though.  While it helps in diagnosing a very specific type of problem
> >(applications that have a maximum allowable latency), its really not useful, and
> >potentially misleading, in the general case.  Specificaly it may show a very
> >large RTO even if that RTO was an erroneous spike in behavior earlier in the
> >lifetime of a given transport, even if that RTO is not representative of the
> >current behavior of the association.  It seems to me like this stat might be
> >better collected using a stap script or by adding a trace point to
> >sctp_transport_update_rto.  If the application needs to know this information
> >internally during its operation to take corrective action, you can already get
> >it via the SCTP_GET_PEER_ADDR_INFO socket option on a per transport basis just
> >as efficiently.

SCTP_GET_PEER_ADDR_INFO doesn't help here as the whole point of this
stat is to get max(rto) as seen by the SCTP stack.

> The max_rto is reset after each getsockopt(), so in effect, the
> application sets its own polling interval and gets the max rto
> achieved during it.  If the rto hasn't changed, then the last value
> is returned.  Not sure how much I like that.  I would rather get max
> rto achieved per polling period and upon reset, max_rto is
> accumulated again (easy way to do that is set to rto_min on reset).
> This way an monitoring thread can truly represent the max rto
> reported by association.  It should normally remain steady, but this
> will show spikes, if any.

I would still reset it to 0 but I agree that it makes more sense to
return 0 if max(rto) remains unchanged within the observation period
rather than returning the previous max(rto).
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