linux-kernel - Re: [PATCH] trace: adjust code layout in get_recursion

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Message-ID: <20170822151410.xroe4mrkk32tlrxv@hirez.programming.kicks-ass.net>
Date:   Tue, 22 Aug 2017 17:14:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] trace: adjust code layout in get_recursion_context

On Tue, Aug 22, 2017 at 04:40:24PM +0200, Jesper Dangaard Brouer wrote:
> In an XDP redirect applications using tracepoint xdp:xdp_redirect to
> diagnose TX overrun, I noticed perf_swevent_get_recursion_context()
> was consuming 2% CPU. This was reduced to 1.6% with this simple
> change.

It is also incorrect. What do you suppose it now returns when the NMI
hits a hard IRQ which hit during a Soft IRQ?

> @@ -208,12 +208,12 @@ static inline int get_recursion_context(int *recursion)
>  {
>  	int rctx;
>  
> +	if (in_softirq())
> +		rctx = 1;
>  	else if (in_irq())
>  		rctx = 2;
> +	else if (in_nmi())
> +		rctx = 3;
>  	else
>  		rctx = 0;
>  
>