linux-kernel - Re: [PATCH] arm64: don't account for cpu offline time with irqsoff tracer

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Date:   Wed, 5 Sep 2018 13:29:05 +0100
From:   Will Deacon <will.deacon@....com>
To:     Zhizhou Zhang <zhizhouzhang@...micro.com>
Cc:     catalin.marinas@....com, james.morse@....com,
        julien.thierry@....com, dave.martin@....com,
        suzuki.poulose@....com, sudeep.holla@....com, adobriyan@...il.com,
        lorenzo.pieralisi@....com, mark.rutland@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: don't account for cpu offline time with irqsoff
 tracer

On Wed, Sep 05, 2018 at 04:19:43PM +0800, Zhizhou Zhang wrote:
> This is no need to account for cpu offline time with irqsoff tracer.
> We can trigger a large irqsoff latency with below commands:
> 
> $ echo irqsoff > /sys/kernel/debug/tracing/current_tracer
> $ echo 0 > /sys/kernel/debug/tracing/options/function-trace
> $ echo 1 > /sys/kernel/debug/tracing/tracing_on
> $ echo 0 > /sys/devices/system/cpu/cpu1/online
> $ # wait a while ...
> $ echo 1 > /sys/devices/system/cpu/cpu1/online
> $ cat /sys/kernel/debug/tracing/trace
> 
> Signed-off-by: Zhizhou Zhang <zhizhouzhang@...micro.com>
> ---
>  arch/arm64/kernel/smp.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 25fcd22..faed8f6 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -346,6 +346,7 @@ void cpu_die(void)
>  	idle_task_exit();
>  
>  	local_daif_mask();
> +	stop_critical_timings();
>  
>  	/* Tell __cpu_die() that this CPU is now safe to dispose of */
>  	(void)cpu_report_death();
> -- 
> 1.9.1

Hmm, so there are only a handful of other callers of stop_critical_timings()
which suggests that we probably shouldn't be calling this from deep in the
arch code. Do other architectures have the same problem? If not, how do they
avoid it?

Will