linux-kernel - Re: [PATCH] perf bench: Add benchmark of find_next

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Date:   Fri, 24 Jul 2020 07:45:03 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] perf bench: Add benchmark of find_next_bit

On Fri, Jul 24, 2020 at 12:19:59AM -0700, Ian Rogers wrote:
> for_each_set_bit, or similar functions like for_each_cpu, may be hot
> within the kernel. If many bits were set then one could imagine on
> Intel a "bt" instruction with every bit may be faster than the function
> call and word length find_next_bit logic. Add a benchmark to measure
> this.
 
> This benchmark on AMD rome and Intel skylakex shows "bt" is not a good
> option except for very small bitmaps.

Small bitmaps is a common case in the kernel (e.g. cpu bitmaps) 

But the current code isn't that great for small bitmaps. It always looks horrific
when I look at PT traces or brstackinsn, especially since it was optimized
purely for code size at some point.

Probably would be better to have different implementations for
different sizes.

-Andi