linux-kernel - Re: [PATCHv2] Introduced new tracing mode KCOV_MODE

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Message-ID: <927385c7-0155-22b0-c2f3-7776b6fe374c@alexander-lochmann.de>
Date:   Sat, 23 Oct 2021 00:03:16 +0200
From:   Alexander Lochmann <info@...xander-lochmann.de>
To:     Peter Zijlstra <peterz@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Andrey Konovalov <andreyknvl@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Andrew Klychkov <andrew.a.klychkov@...il.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Johannes Berg <johannes@...solutions.net>,
        Ingo Molnar <mingo@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Jakub Kicinski <kuba@...nel.org>,
        Aleksandr Nogikh <nogikh@...gle.com>,
        kasan-dev@...glegroups.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2] Introduced new tracing mode KCOV_MODE_UNIQUE.

Maybe Dmitry can shed some light on this. He actually suggested that 
optimization.

- Alex

On 29.09.21 10:33, Peter Zijlstra wrote:
> On Mon, Sep 27, 2021 at 07:33:40PM +0200, Alexander Lochmann wrote:
>> The existing trace mode stores PCs in execution order. This could lead
>> to a buffer overflow if sufficient amonut of kernel code is executed.
>> Thus, a user might not see all executed PCs. KCOV_MODE_UNIQUE favors
>> completeness over execution order. While ignoring the execution order,
>> it marks a PC as exectued by setting a bit representing that PC. Each
>> bit in the shared buffer represents every fourth byte of the text
>> segment.  Since a call instruction on every supported architecture is
>> at least four bytes, it is safe to just store every fourth byte of the
>> text segment.
> 
> I'm still trying to wake up, but why are call instruction more important
> than other instructions? Specifically, I'd think any branch instruction
> matters for coverage.
> 
> More specifically, x86 can do a tail call with just 2 bytes.
> 

-- 
Alexander Lochmann                PGP key: 0xBC3EF6FD
Heiliger Weg 72                   phone:  +49.231.28053964
D-44141 Dortmund                  mobile: +49.151.15738323



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