linux-kernel - Re: [RFC 1/2] x86/tsc: add a timer to make sure tsc

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Message-ID: <87blam2d0x.ffs@nanos.tec.linutronix.de>
Date:   Sat, 10 Apr 2021 20:46:38 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, rui.zhang@...el.com,
        andi.kleen@...el.com, dave.hansen@...el.com, len.brown@...el.com
Subject: Re: [RFC 1/2] x86/tsc: add a timer to make sure tsc_adjust is always checked

Feng,

On Sat, Apr 10 2021 at 22:38, Feng Tang wrote:
> On Sat, Apr 10, 2021 at 11:27:11AM +0200, Thomas Gleixner wrote:
>> > +static int __init start_sync_check_timer(void)
>> > +{
>> > +	if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
>> > +		return 0;
>> > +
>> > +	timer_setup(&tsc_sync_check_timer, tsc_sync_check_timer_fn, 0);
>> > +	tsc_sync_check_timer.expires = jiffies + SYNC_CHECK_INTERVAL;
>> > +	add_timer(&tsc_sync_check_timer);
>> > +
>> > +	return 0;
>> > +}
>> > +late_initcall(start_sync_check_timer);
>> 
>> So right now, if someone adds 'tsc=reliable' on the kernel command line
>> then all of the watchdog checking, except for the idle enter TSC_ADJUST
>> check is disabled. The NOHZ full people are probably going to be pretty
>> unhappy about yet another unconditional timer they have to chase down.
>> 
>> So this needs some more thought.
>
> 'tsc=reliable' in cmdline will set 'tsc_clocksource_reliable' to 1, so
> we can skip starting this timer if 'tsc_clocksource_reliable==1' ?

Then we can just ignore that patch alltogether because of 2/2 doing:

+	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
+	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
+	    boot_cpu_has(X86_FEATURE_TSC_ADJUST) &&
+	    nr_online_nodes <= 2)
+		tsc_clocksource_reliable = 1;

....

I said for a reason:

>> So this needs some more thought.

Thanks,

        tglx