linux-kernel - Re: [PATCH AUTOSEL 5.10 2/2] timekeeping: Always check for negative motion

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Message-ID: <Z02i3MxY7JEJfgQ4@duo.ucw.cz>
Date: Mon, 2 Dec 2024 13:06:52 +0100
From: Pavel Machek <pavel@...x.de>
To: Sasha Levin <sashal@...nel.org>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <jstultz@...gle.com>, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, rdunlap@...radead.org,
	paulmck@...nel.org
Subject: Re: [PATCH AUTOSEL 5.10 2/2] timekeeping: Always check for negative
 motion

Hi!

> [ Upstream commit c163e40af9b2331b2c629fd4ec8b703ed4d4ae39 ]
> 
> clocksource_delta() has two variants. One with a check for negative motion,
> which is only selected by x86. This is a historic leftover as this function
> was previously used in the time getter hot paths.
> 
> Since 135225a363ae timekeeping_cycles_to_ns() has unconditional protection
> against this as a by-product of the protection against 64bit math
> overflow.
> timekeeping_advance(). The extra conditional there is not hurting anyone.

We don't have 135225a363ae in 5.10. So we probably should not have
this?

Best regards,
								Pavel

> +++ b/arch/x86/Kconfig
> @@ -107,7 +107,6 @@ config X86
>  	select ARCH_WANTS_THP_SWAP		if X86_64
>  	select BUILDTIME_TABLE_SORT
>  	select CLKEVT_I8253
> -	select CLOCKSOURCE_VALIDATE_LAST_CYCLE
>  	select CLOCKSOURCE_WATCHDOG
>  	select DCACHE_WORD_ACCESS
>  	select EDAC_ATOMIC_SCRUB
> diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
> index a09b1d61df6a5..5cbedc0a06efc 100644
> --- a/kernel/time/Kconfig
> +++ b/kernel/time/Kconfig
> @@ -17,11 +17,6 @@ config ARCH_CLOCKSOURCE_DATA
>  config ARCH_CLOCKSOURCE_INIT
>  	bool
>  
> -# Clocksources require validation of the clocksource against the last
> -# cycle update - x86/TSC misfeature
> -config CLOCKSOURCE_VALIDATE_LAST_CYCLE
> -	bool
> -
>  # Timekeeping vsyscall support
>  config GENERIC_TIME_VSYSCALL
>  	bool
> diff --git a/kernel/time/timekeeping_internal.h b/kernel/time/timekeeping_internal.h
> index 4ca2787d1642e..1d4854d5c386e 100644
> --- a/kernel/time/timekeeping_internal.h
> +++ b/kernel/time/timekeeping_internal.h
> @@ -15,7 +15,6 @@ extern void tk_debug_account_sleep_time(const struct timespec64 *t);
>  #define tk_debug_account_sleep_time(x)
>  #endif
>  
> -#ifdef CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE
>  static inline u64 clocksource_delta(u64 now, u64 last, u64 mask)
>  {
>  	u64 ret = (now - last) & mask;
> @@ -26,12 +25,6 @@ static inline u64 clocksource_delta(u64 now, u64 last, u64 mask)
>  	 */
>  	return ret & ~(mask >> 1) ? 0 : ret;
>  }
> -#else
> -static inline u64 clocksource_delta(u64 now, u64 last, u64 mask)
> -{
> -	return (now - last) & mask;
> -}
> -#endif
>  
>  /* Semi public for serialization of non timekeeper VDSO updates. */
>  extern raw_spinlock_t timekeeper_lock;

-- 
DENX Software Engineering GmbH,        Managing Director: Erika Unter
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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