linux-kernel - Re: [PATCH v3 5/5] arm64: barrier: Handle waiting in smp_cond_load_relaxed

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Message-ID: <e2e8788d-86b4-092a-37f5-286b776cc061@gentwo.org>
Date: Mon, 30 Jun 2025 09:33:43 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org, 
    linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org, arnd@...db.de, 
    catalin.marinas@....com, will@...nel.org, peterz@...radead.org, 
    akpm@...ux-foundation.org, mark.rutland@....com, harisokn@...zon.com, 
    ast@...nel.org, memxor@...il.com, zhenglifeng1@...wei.com, 
    xueshuai@...ux.alibaba.com, joao.m.martins@...cle.com, 
    boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v3 5/5] arm64: barrier: Handle waiting in
 smp_cond_load_relaxed_timewait()

On Thu, 26 Jun 2025, Ankur Arora wrote:

> @@ -222,6 +223,53 @@ do {									\
>  #define __smp_timewait_store(ptr, val)					\
>  		__cmpwait_relaxed(ptr, val)
>
> +/*
> + * Redefine ARCH_TIMER_EVT_STREAM_PERIOD_US locally to avoid include hell.
> + */
> +#define __ARCH_TIMER_EVT_STREAM_PERIOD_US 100UL
> +extern bool arch_timer_evtstrm_available(void);
> +
> +static inline u64 ___smp_cond_spinwait(u64 now, u64 prev, u64 end,
> +				       u32 *spin, bool *wait, u64 slack);
> +/*
> + * To minimize time spent spinning, we want to allow a large overshoot.
> + * So, choose a default slack value of the event-stream period.
> + */
> +#define SMP_TIMEWAIT_DEFAULT_US __ARCH_TIMER_EVT_STREAM_PERIOD_US
> +
> +static inline u64 ___smp_cond_timewait(u64 now, u64 prev, u64 end,
> +				       u32 *spin, bool *wait, u64 slack)
> +{
> +	bool wfet = alternative_has_cap_unlikely(ARM64_HAS_WFXT);
> +	bool wfe, ev = arch_timer_evtstrm_available();

An unitialized and initialized variable on the same line. Maybe separate
that. Looks confusing and unusual to me.

> +	u64 evt_period = __ARCH_TIMER_EVT_STREAM_PERIOD_US;
> +	u64 remaining = end - now;
> +
> +	if (now >= end)
> +		return 0;
> +	/*
> +	 * Use WFE if there's enough slack to get an event-stream wakeup even
> +	 * if we don't come out of the WFE due to natural causes.
> +	 */
> +	wfe = ev && ((remaining + slack) > evt_period);

The line above does not matter for the wfet case and the calculation is
ignored. We hope that in the future wfet will be the default case.