linux-kernel - Re: [PATCH v9 01/15] asm-generic: add barrier smp_cond_load_relaxed

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Message-ID: <9cecd8a5-82e5-69ef-502b-45219a45006b@gentwo.org>
Date: Thu, 7 Nov 2024 18:33:26 -0800 (PST)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
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Subject: Re: [PATCH v9 01/15] asm-generic: add barrier
 smp_cond_load_relaxed_timeout()

On Thu, 7 Nov 2024, Ankur Arora wrote:

> +#ifndef smp_cond_time_check_count
> +/*
> + * Limit how often smp_cond_load_relaxed_timeout() evaluates time_expr_ns.
> + * This helps reduce the number of instructions executed while spin-waiting.
> + */
> +#define smp_cond_time_check_count	200
> +#endif

I dont like these loops that execute differently depending on the
hardware. Can we use cycles and ns instead to have defined periods of
time? Later patches establish the infrastructure to convert cycles to
nanoseconds and microseconds. Use that?

> +#ifndef smp_cond_load_relaxed_timeout
> +#define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_expr_ns,	\
> +				      time_limit_ns) ({			\
> +	typeof(ptr) __PTR = (ptr);					\
> +	__unqual_scalar_typeof(*ptr) VAL;				\
> +	unsigned int __count = 0;					\
> +	for (;;) {							\
> +		VAL = READ_ONCE(*__PTR);				\
> +		if (cond_expr)						\
> +			break;						\
> +		cpu_relax();						\
> +		if (__count++ < smp_cond_time_check_count)		\
> +			continue;					\
> +		if ((time_expr_ns) >= time_limit_ns)			\
> +			break;						\

Calling the clock retrieval function repeatedly should be fine and is
typically done in user space as well as in kernel space for functions that
need to wait short time periods.