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From: Alexander Lochmann <info@alexander-lochmann.de>
To: Peter Zijlstra <peterz@infradead.org>,
Dmitry Vyukov <dvyukov@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>,
Jonathan Corbet <corbet@lwn.net>,
Andrew Klychkov <andrew.a.klychkov@gmail.com>,
Miguel Ojeda <ojeda@kernel.org>,
Randy Dunlap <rdunlap@infradead.org>,
Johannes Berg <johannes@sipsolutions.net>,
Ingo Molnar <mingo@kernel.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Jakub Kicinski <kuba@kernel.org>,
Aleksandr Nogikh <nogikh@google.com>,
kasan-dev@googlegroups.com, linux-doc@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCHv2] Introduced new tracing mode KCOV_MODE_UNIQUE.
Date: Sat, 23 Oct 2021 00:03:16 +0200 [thread overview]
Message-ID: <927385c7-0155-22b0-c2f3-7776b6fe374c@alexander-lochmann.de> (raw)
In-Reply-To: <YVQkzCryS9dkvRGB@hirez.programming.kicks-ass.net>
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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
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next prev parent reply other threads:[~2021-10-22 22:13 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-27 17:33 Alexander Lochmann
2021-09-29 8:33 ` Peter Zijlstra
2021-10-22 22:03 ` Alexander Lochmann [this message]
2021-10-23 7:01 ` Dmitry Vyukov
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