linux-kernel - Re: [PATCH 4/6] perf: Optimize get_recursion

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Date:   Fri, 30 Oct 2020 18:11:38 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, rostedt@...dmis.org,
        linux-kernel@...r.kernel.org, kan.liang@...ux.intel.com,
        acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, ak@...ux.intel.com, eranian@...gle.com,
        brouer@...hat.com
Subject: Re: [PATCH 4/6] perf: Optimize get_recursion_context()

On Fri, 30 Oct 2020 16:13:49 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

>   "Look ma, no branches!"
> 
> Cc: Jesper Dangaard Brouer <brouer@...hat.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---

Cool trick! :-)

Acked-by: Jesper Dangaard Brouer <brouer@...hat.com>

>  kernel/events/internal.h |   17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -205,16 +205,15 @@ DEFINE_OUTPUT_COPY(__output_copy_user, a
>  
>  static inline int get_recursion_context(int *recursion)
>  {
> -	int rctx;
> +	unsigned int pc = preempt_count();
> +	unsigned int rctx = 0;
>  
> -	if (unlikely(in_nmi()))
> -		rctx = 3;
> -	else if (in_irq())
> -		rctx = 2;
> -	else if (in_serving_softirq())
> -		rctx = 1;
> -	else
> -		rctx = 0;
> +	if (pc & (NMI_MASK))
> +		rctx++;
> +	if (pc & (NMI_MASK | HARDIRQ_MASK))
> +		rctx++;
> +	if (pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET))
> +		rctx++;
>  
>  	if (recursion[rctx])
>  		return -1;
> 
> 



-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer