netdev - Re: [net-next PATCH v2 5/8] net: Track start of busy loop instead of when it should end

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Date:   Fri, 24 Mar 2017 08:48:13 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [net-next PATCH v2 5/8] net: Track start of busy loop instead of
 when it should end

On Fri, Mar 24, 2017 at 4:16 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2017-03-23 at 22:55 -0700, Alexander Duyck wrote:
>
>> Right, but time_after assumes roll over.  When you are using a time
>> value based off of local_clock() >> 10, you don't ever roll over when
>> you do addition.  Just the clock rolls over.  At least on 64 bit
>> systems.
>>
>> So if local time approaches something like all 1's, and we have
>> shifted it by 10 it is then the max it can ever reach is
>> 0x003FFFFFFFFFFFFF.  I can add our loop time to that and it won't roll
>> over.  In the mean time the busy_loop_us_ can never exceed whatever I
>> added to that so we are now locked into a loop.  I realize I am
>> probably being pedantic, and it will have an exceedingly small rate of
>> occurrence, but it is still an issue.
>
> Do you realize that a 64bit clock wont rollover before the host has
> reached 584 years of uptime ?

Yeah, that is what I meant by "probably being pedantic".  I was being
a too much of a perfectionist.

So I can work with the ">> 10" approach.  The only thing I think I may
still want to change is that on 32b systems I will still use the
do_procintvec_minmax for busy_poll and busy_read to prevent us from
inputting values less than 0. For 64b systems we can do_procuintvec.
It isn't so much that I don't trust root, it is just that we didn't
really document the ranges anywhere for this so I figure if we at
least lock that down to the usable ranges since root may not be aware
of the implementation details.