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

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Date:   Sat, 31 Oct 2020 00:01:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>, mingo@...nel.org,
        tglx@...utronix.de, 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
Subject: Re: [PATCH 4/6] perf: Optimize get_recursion_context()

On Fri, Oct 30, 2020 at 04:22:48PM -0400, Steven Rostedt wrote:
> As this is something that ftrace recursion also does, perhaps we should
> move this into interrupt.h so that anyone that needs a counter can get
> it quickly, and not keep re-implementing it.

Works for me, however:

> /*
>  * Quickly find what context you are in.
>  * 0 - normal
>  * 1 - softirq
>  * 2 - hard interrupt
>  * 3 - NMI
>  */
> static inline int irq_context()
> {
> 	unsigned int pc = preempt_count();
> 	int rctx = 0;

unsigned

> 
> 	if (pc & (NMI_MASK))
> 		rctx++;
> 	if (pc & (NMI_MASK | HARDIRQ_MASK))
> 		rctx++;
> 	if (pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET))
> 		rctx++;
> 
> 	return rctx;
> }

otherwise you'll get an extra instruction to sign extend it, which is
daft (yes, i've been staring at GCC output far too much).

Also, gcc-9 does worse (like 1 byte iirc) with:

	rctx += !!(pc & (NMI_MASK));
	rctx += !!(pc & (NMI_MASK | HARDIRQ_MASK));
	rctx += !!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET));

but gcc-10 doesn't seem to care.