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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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547 lines
35 KiB
HTML
547 lines
35 KiB
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<meta name="description" content="The perfect emulation setup to study and develop the <<linux-kernel>> v5.9.2, kernel modules, <<qemu-buildroot-setup,QEMU>>, <<gem5-buildroot-setup,gem5>> and x86_64, ARMv7 and ARMv8 <<userland-assembly,userland>> and <<baremetal-setup,baremetal>> assembly, <<c,ANSI C>>, <<cpp,C++>> and <<posix,POSIX>>. EVERYTHING is built from source. <<gdb>> and <<kgdb>> just work. Powered by <<about-the-qemu-buildroot-setup,Buildroot>> and <<about-the-baremetal-setup,crosstool-NG>>. Highly automated. Thoroughly documented. Automated <<test-this-repo,tests>>. "Tested" in an Ubuntu 20.04 host.">
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<title>Linux Kernel Module Cheat</title>
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</head>
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<body id="cache-coherence" class="article">
|
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<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect3">
|
||
<h4 id="cache-coherence"><a class="anchor" href="#cache-coherence"></a><a class="link" href="#cache-coherence">37.6.1. Cache coherence</a></h4>
|
||
<div class="paragraph">
|
||
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|
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</div>
|
||
<div class="paragraph">
|
||
<p>In simple terms, when a certain group of caches of different CPUs are coherent, reads on one core always see the writes previously made by other cores. TODO: is it that strict, or just ordering? TODO what about simultaneous read and writes?</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p><a href="http://www.inf.ed.ac.uk/teaching/courses/pa/Notes/lecture07-sc.pdf" class="bare">http://www.inf.ed.ac.uk/teaching/courses/pa/Notes/lecture07-sc.pdf</a> mentions that:</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Cache coherence:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p>guarantees eventual write propagation</p>
|
||
</li>
|
||
<li>
|
||
<p>guarantees a single order of all writes to same location</p>
|
||
</li>
|
||
<li>
|
||
<p>no guarantees on when writes propagate</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>And notably it contrasts that with <a href="memory-consistency.html">Memory consistency</a>, which according to them is about ordering requirements on <em>different</em> addresses.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Algorithms to keep the caches of different cores of a system coherent. Only matters for multicore systems.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The main goal of such systems is to reduce the number of messages that have to be sent on the coherency bus, and even more importantly, to memory (which passes first through the coherency bus).</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The main software use case example to have in mind is that of multiple threads incrementing an atomic counter as in <a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/cpp/atomic/std_atomic.cpp">userland/cpp/atomic/std_atomic.cpp</a>, see also: <a href="atomic-cpp.html">atomic.cpp</a>. Then, if one processors writes to the cache, other processors have to know about it before they read from that address.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Even if caches are coherent, this is still not enough to avoid data race conditions, because this does not enforce atomicity of read modify write sequences. This is for example shown at: <a href="detailed-gem5-analysis-of-how-data-races-happen.html">Detailed gem5 analysis of how data races happen</a>.</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="memory-consistency.html">Memory consistency</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="can-caches-snoop-data-from-other-caches.html">Can caches snoop data from other caches?</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="vi-cache-coherence-protocol.html">VI cache coherence protocol</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="msi-cache-coherence-protocol.html">MSI cache coherence protocol</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="mesi-cache-coherence-protocol.html">MESI cache coherence protocol</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="mosi-cache-coherence-protocol.html">MOSI cache coherence protocol</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="moesi.html">MOESI cache coherence protocol</a></p>
|
||
</li>
|
||
</ul>
|
||
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|
||
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|
||
<div class="paragraph nav-footer">
|
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<p>↑ Up: <a href="caches.html">Caches</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="memory-consistency.html">Memory consistency</a> →</p>
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