Re: [PATCH v3] printk: fix zero-valued printk timestamps in early boot

Peter Zijlstra
2026-03-30T12:05:50Z

On Mon, Mar 30, 2026 at 12:42:59AM +0200, Thomas Gleixner wrote:
> Quite the contrary, I'm going to get rid of technical debt:
>
> The get_cycles() related changes in tsc.h are going to end up in a
> obviously revised formal patch tomorrow as there is exactly _zero_
> requirement to provide this as a functional interface.
>
> 1) The default implementation in asm-generic returns 0
>
> Ergo any code depending on a functional implementation is broken
> by definition.
>
> 2) The ops/cycles metrics are as bogus as the infamous BogoMIPS metrics
>
> The "cycle" counter runs on almost all contemporary CPUs at a fixed
> frequency, which is completely unrelated to the actual CPU
> frequency and therefore to the actual CPU cycles.
>
> The only realistic metric is ops/sec and nothing else and that can
> be trivially achieved by using the generic time getter interfaces.
>
> Those might end up resulting in bogus benchmark results too if the
> underlying clocksource is jiffies, but that's again a matter of
> accepting reality.
>
> If people really mandate that ops/bogo_cycles is required for the
> very wrong reasons, then I'm happy to bring it back with a global
> name change from get_cycles() to get_bogo_cycles() which excludes
> it from any serious usage including printk.
>
> Thanks,
>
> tglx
> ---
> arch/x86/include/asm/tsc.h | 11 ++--
> arch/x86/kernel/platform-quirks.c | 2
> arch/x86/kernel/tsc.c | 96 ++++++++++++++++++++++++++++++--------
> 3 files changed, 85 insertions(+), 24 deletions(-)
>
> --- a/arch/x86/include/asm/tsc.h
> +++ b/arch/x86/include/asm/tsc.h
> @@ -76,10 +76,7 @@ extern void disable_TSC(void);
>
> static inline cycles_t get_cycles(void)
> {
> - if (!IS_ENABLED(CONFIG_X86_TSC) &&
> - !cpu_feature_enabled(X86_FEATURE_TSC))
> - return 0;
> - return rdtsc();
> + return 0;
> }
> #define get_cycles get_cycles
>

So this is probably going to break a bunch of generic stuff. There is a
fair amount of 'get_cycles' usage out in the tree.

Mostly it appears to be various test code.

But I do agree that get_cycles() is terrible and should go away.