From:   Thomas Gleixner <tglx@linutronix.de>
To:     Waiman Long <longman@redhat.com>, paulmck@kernel.org
Cc:     linux-kernel@vger.kernel.org, john.stultz@linaro.org,
        sboyd@kernel.org, corbet@lwn.net, Mark.Rutland@arm.com,
        maz@kernel.org, kernel-team@meta.com, neeraju@codeaurora.org,
        ak@linux.intel.com, feng.tang@intel.com, zhengjun.xing@intel.com,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>, x86@kernel.org
Subject: Re: [PATCH v2 clocksource 6/7] clocksource: Verify HPET and PMTMR
 when TSC unverified
In-Reply-To: <39752908-cc10-d63f-d02e-381693060af8@redhat.com>
References: <20230125002708.GA1471122@paulmck-ThinkPad-P17-Gen-1>
 <20230125002730.1471349-6-paulmck@kernel.org> <87wn51znsh.ffs@tglx>
 <15e8c929-845e-ef65-dc04-a51f071dd256@redhat.com>
 <20230201195517.GM2948950@paulmck-ThinkPad-P17-Gen-1>
 <39752908-cc10-d63f-d02e-381693060af8@redhat.com>
Date:   Thu, 02 Feb 2023 08:57:39 +0100
Message-ID: <87h6w4zeh8.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Wed, Feb 01 2023 at 22:40, Waiman Long wrote:
> On 2/1/23 14:55, Paul E. McKenney wrote:
>>>>> Therefore, when watchdog verification of TSC is disabled, enable it for
>>>>> HPET and PMTMR (AKA ACPI PM timer).  This provides the needed in-kernel
>>>>> time-skew diagnostic without degrading the system's performance.
>>>> I'm more than unhappy about this. We finally have a point where the TSC
>>>> watchdog overhead can go away without adding TSC=reliable to the kernel
>>>> commandline.
>>>>
>>>> Now you add an unconditionally enforce the watchdog again in a way which
>>>> even cannot be disabled on the kernel command line.
>>>>
>>>> Patently bad idea, no cookies for you!
>>> I have a similar concern about this patch as well. That is why I was
>>> suggesting to have this enabled for a limited time after boot for sanity
>>> checking purpose only.
>> Fair enough!
>>
>> If the watchdog checking of HPET and/or PMTMR against TSC only happens
>> only when the sysadm asks for it, would you still want to have the ability
>> to enable such watchdog checking at boot time, and then to disable it
>> once the system had been running for some limited time?
>
> Yes, being optional is another way to avoid the overhead for the 
> majority of users. The paranoids can turn it on if they want to.

Yes, opt-in is good enough.

Thanks,

        tglx