linux-kernel - Re: [RFC PATCH-tip v4 10/10] locking/rwsem: Add a boot parameter to reader spinning threshold

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Date:   Wed, 24 Aug 2016 14:39:11 -0400
From:   Waiman Long <waiman.long@....com>
To:     Davidlohr Bueso <dave@...olabs.net>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
        <x86@...nel.org>, <linux-alpha@...r.kernel.org>,
        <linux-ia64@...r.kernel.org>, <linux-s390@...r.kernel.org>,
        <linux-arch@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        Jason Low <jason.low2@...com>,
        Dave Chinner <david@...morbit.com>,
        Jonathan Corbet <corbet@....net>,
        Scott J Norton <scott.norton@....com>,
        Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip v4 10/10] locking/rwsem: Add a boot parameter
 to reader spinning threshold

On 08/24/2016 12:00 AM, Davidlohr Bueso wrote:
> On Thu, 18 Aug 2016, Waiman Long wrote:
>
>> The default reader spining threshold is current set to 4096. However,
>> the right reader spinning threshold may vary from one system to
>> another and among the different architectures. This patch adds a new
>> kernel boot parameter to modify the threshold value. This enables
>> better tailoring to the needs of different systems as well as for
>> testing purposes.
>
> It also means that those systems could very easily be incorrectly 
> tailored.
> (and worse case scenario: reboot, is obviously a terrible way to get rid
> of any issues). I very much disagree with exposing this sort of core 
> stuff,
> it should work well for everyone out of the box, not relying on users to
> properly configure this.
>
> Thanks,
> Davidlohr

I also have some concern about exposing this kernel parameter as it will 
be hard to tune. That is why I put it at the end to gauge the opinion of 
others. I will leave this out when I send out the next version.

Cheers,
Longman