netdev - Re: [PATCH net-next] ipv6: only update __use and lastusetime once per jiffy at most

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Message-ID: <20171015181918.ri2gjp2ypdm37pwd@kafai-mbp>
Date:   Sun, 15 Oct 2017 11:19:49 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Eric Dumazet <edumazet@...gle.com>
CC:     Wei Wang <weiwan@...gle.com>, David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next] ipv6: only update __use and lastusetime once
 per jiffy at most

On Sat, Oct 14, 2017 at 12:26:28AM +0000, Eric Dumazet wrote:
> On Fri, Oct 13, 2017 at 5:09 PM, Martin KaFai Lau <kafai@...com> wrote:
> > On Fri, Oct 13, 2017 at 10:08:07PM +0000, Wei Wang wrote:
> >> From: Wei Wang <weiwan@...gle.com>
> >>
> >> In order to not dirty the cacheline too often, we try to only update
> >> dst->__use and dst->lastusetime at most once per jiffy.
> >
> >
> >> As dst->lastusetime is only used by ipv6 garbage collector, it should
> >> be good enough time resolution.
> > Make sense.
> >
> >> And __use is only used in ipv6_route_seq_show() to show how many times a
> >> dst has been used. And as __use is not atomic_t right now, it does not
> >> show the precise number of usage times anyway. So we think it should be
> >> OK to only update it at most once per jiffy.
> > If __use is only bumped HZ number of times per second and we can do ~3Mpps now,
> > would __use be way off?
> 
> It is not used in the kernel, and is not even reported by user space
> (iproute2) currently.
> 
> With the percpu stuff, we never did the sum anyway.
The pcpu_rt currently does not track __use.

> 
> I believe we should be fine by being very lazy on this field.
> 
> If really someones complain, we will see, but insuring ~one update per
> HZ seems fine.
Fair point.  Make sense.
We currently also don't find the ipv6_route proc-file very useful
other than debugging purpose.