linux-kernel - RE: [PATCH v2 1/3] x86/cpufeatures: Enumerate user wait instructions

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Date:   Thu, 21 Feb 2019 22:57:49 +0000
From:   "Yu, Fenghua" <fenghua.yu@...el.com>
To:     "Yu, Fenghua" <fenghua.yu@...el.com>,
        Andy Lutomirski <luto@...nel.org>
CC:     "Xu, Tao3" <tao3.xu@...el.com>,
        "Liu, Jingqi" <jingqi.liu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "Ingo Molnar" <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: RE: [PATCH v2 1/3] x86/cpufeatures: Enumerate user wait instructions

> From: Fenghua Yu [mailto:fenghua.yu@...el.com]
> On Wed, Feb 20, 2019 at 10:37:27PM -0800, Andy Lutomirski wrote:
> Or to simplify the situation, how about we still use zero as global max wait
> time (i.e. no limitation for global wait time) as implemented in this patch
> set? User can still change the value to any value based on their usage. Isn't
> this a right way to take care of any usage?

Plus, if scheduler timers works, umwait will be forced time out by timer in HZ anyway.

Only for case without scheduler timer, sysadmin may need to set a small max umwait time to force timout. But in this case (real time), I doubt user app really wants to call umwait to save power with long latency of waking up from umwait. So likely in this case, user app won't call umwait and there is no usage to set a small value for max umwait time. 

Thanks.

-Fenghua