Subject: [PATCH v3 2/2] arm64: use WFE for long delays

From: [email-masked] (Will Deacon)
Date: Wed, 11 Oct 2017 16:13:35 +0100

Hi Julien,

On Fri, Sep 29, 2017 at 11:52:30AM +0100, Julien Thierry wrote:
> The current delay implementation uses the yield instruction, which is a
> hint that it is beneficial to schedule another thread. As this is a hint,
> it may be implemented as a NOP, causing all delays to be busy loops. This
> is the case for many existing CPUs.
> 
> Taking advantage of the generic timer sending periodic events to all
> cores, we can use WFE during delays to reduce power consumption. This is
> beneficial only for delays longer than the period of the timer event
> stream.
> 
> If timer event stream is not enabled, delays will behave as yield/busy
> loops.
> 
> Signed-off-by: Julien Thierry <[email-masked]>
> Cc: Catalin Marinas <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> Cc: Mark Rutland <[email-masked]>
> ---
>  arch/arm64/lib/delay.c               | 23 +++++++++++++++++++----
>  include/clocksource/arm_arch_timer.h |  4 +++-
>  2 files changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/lib/delay.c b/arch/arm64/lib/delay.c
> index dad4ec9..4dc27f3 100644
> --- a/arch/arm64/lib/delay.c
> +++ b/arch/arm64/lib/delay.c
> @@ -24,10 +24,28 @@
>  #include <linux/module.h>
>  #include <linux/timex.h>
> 
> +#include <clocksource/arm_arch_timer.h>
> +
> +#define USECS_TO_CYCLES(TIME_USECS)			\
> +	xloops_to_cycles((TIME_USECS) * 0x10C7UL)

The macro parameter can be lower-case here.

> +static inline unsigned long xloops_to_cycles(unsigned long xloops)
> +{
> +	return (xloops * loops_per_jiffy * HZ) >> 32;
> +}
> +
>  void __delay(unsigned long cycles)
>  {
>  	cycles_t start = get_cycles();
> 
> +	if (arch_timer_evtstrm_available()) {

Hmm, is this never called in a context where preemption is enabled?
Maybe arch_timer_evtstrm_available should be using raw_smp_processor_id()
under the hood.

> +		const cycles_t timer_evt_period =
> +			USECS_TO_CYCLES(ARCH_TIMER_EVT_STREAM_PERIOD_US);
> +
> +		while ((get_cycles() - start + timer_evt_period) < cycles)
> +			wfe();
> +	}
> +
>  	while ((get_cycles() - start) < cycles)
>  		cpu_relax();
>  }
> @@ -35,10 +53,7 @@ void __delay(unsigned long cycles)
> 
>  inline void __const_udelay(unsigned long xloops)
>  {
> -	unsigned long loops;
> -
> -	loops = xloops * loops_per_jiffy * HZ;
> -	__delay(loops >> 32);
> +	__delay(xloops_to_cycles(xloops));
>  }
>  EXPORT_SYMBOL(__const_udelay);
> 
> diff --git a/include/clocksource/arm_arch_timer.h b/include/clocksource/arm_arch_timer.h
> index 4e28283..349e595 100644
> --- a/include/clocksource/arm_arch_timer.h
> +++ b/include/clocksource/arm_arch_timer.h
> @@ -67,7 +67,9 @@ enum arch_timer_spi_nr {
>  #define ARCH_TIMER_USR_VT_ACCESS_EN	(1 << 8) /* virtual timer registers */
>  #define ARCH_TIMER_USR_PT_ACCESS_EN	(1 << 9) /* physical timer registers */
> 
> -#define ARCH_TIMER_EVT_STREAM_FREQ	10000	/* 100us */
> +#define ARCH_TIMER_EVT_STREAM_PERIOD_US	100
> +#define ARCH_TIMER_EVT_STREAM_FREQ				\
> +	(USEC_PER_SEC / ARCH_TIMER_EVT_STREAM_PERIOD_US)

This needs an ack from Marc or Mark.

Will