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<p class="MsoNormal"><span style="font-size:12.0pt;color:#1F497D">Hi Mathieu,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:#1F497D">Thanks for clarifying the restrictions of ‘shm-path’ option!<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:#1F497D">Our purpose is to use exclusively reserved memory for tracing. Our customers request a certain amount of free memory on the system for future development. In development stage chunks
of this “free” memory are grabbed by the applications and tracing lacks memory. We want to allocate this “free” memory in an early boot stage to ensure that we get the amount of memory we need for tracing. E.g. if we have reserved 512 Mbyte of memory, we know
that we can start lttng at a later point of time with 16 buffers of 4 Mbyte on a 8 core system. But this works only, if we can provide the previously reserved memory to lttng for buffer allocation.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:#1F497D">Is there some general concern about using POSIX shared memory for buffers in kernel tracing mode (apart from performance reasons)?
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:#1F497D">I used lttng-crash only for testing, because I wanted to verify, if tracing data is available from provided shared memory. If this is ensured, I agree with you, that ‘lttng snapshot’
can do the work.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:#1F497D">Regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:#1F497D">Thomas
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="mso-fareast-language:DE">From:</span></b><span lang="EN-US" style="mso-fareast-language:DE"> Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
<br>
<b>Sent:</b> Montag, 4. März 2019 17:01<br>
<b>To:</b> Schickentanz, Thomas <thomas.schickentanz@harman.com><br>
<b>Cc:</b> lttng-dev <lttng-dev@lists.lttng.org><br>
<b>Subject:</b> [EXTERNAL] Re: [lttng-dev] Tracing directly into shared memory<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">----- On Mar 4, 2019, at 4:28 AM, Thomas Schickentanz <<a href="mailto:thomas.schickentanz@harman.com">thomas.schickentanz@harman.com</a>>
wrote:<br>
<br>
<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Hi all,</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:black">My idea is to pre-allocate memory on system startup which can later be used by lttng for tracing. It should work like flight recorder mode without extracting data from ringbuffers
to trace file while recording data. Data extraction of shm ringbuffers should be done when recording has been finished.</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:black">As per documention with option
</span><span lang="EN-US" style="font-size:12.0pt;font-family:"Courier New";color:black">`--shm-path‘</span><span lang="EN-US" style="font-size:12.0pt;color:black"> ringbuffer for tracing can be created in shared memory. With
</span><span lang="EN-US" style="font-size:12.0pt;font-family:"Courier New";color:black">lttng-crash</span><span lang="EN-US" style="font-size:12.0pt;color:black"> data can be extracted from ringbuffer.</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:black">Can this option be used with shared memory in volatile memory (default ram), too? And is it possible to record kernel space traces with this option?</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:black">I tried
</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;font-family:"Courier New";color:black">lttng create testshm --shm-path=/dev/shm --snapshot</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;font-family:"Courier New";color:black">lttng enable-channel c0 -s testshm -k</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;font-family:"Courier New";color:black">lttng enable-event -s testshm -c c0 -k -a</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;font-family:"Courier New";color:black">lttng start</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;font-family:"Courier New";color:black">lttng stop</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;font-family:"Courier New";color:black">lttng-crash -extract=/home/test /dev/shm</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:black">With lttng and lttng-crash everything seems to work fine (no error messages).</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:black">However the extract folder keeps empty.</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:12.0pt;color:black">Has someone researched into this option and can help to clarify the points?</span><span style="color:black"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">Hi Thomas,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">About "--shm-path" and lttng-crash, those are discussed here:
<a href="https://clicktime.symantec.com/3YZ15pbJVSD47yeXxhXiwqE7Vc?u=https%3A%2F%2Flttng.org%2Fdocs%2Fv2.10%2F%23doc-persistent-memory-file-systems">
https://lttng.org/docs/v2.10/#doc-persistent-memory-file-systems</a><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">I'm not sure I completely understand your use-case and whether your intent is to extract trace data after a<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">system crash or if the "snapshot" mode is enough for your needs.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">Note that the "shm-path" option only applies to UST (user-space) traces at the moment. It has not been implemented<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">for kernel tracing yet.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">The way we typically use it for extraction of trace data from user-space when a system crashes is to use the pmem driver to<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">create a block device out of a range of the system memory, and then use ext2 or ext4 with the DAX mount option to ensure<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">the filesystem bypasses the page cache.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">Then we ensure that kexec is used to boot a new kernel on crash, or make sure the BIOS does not erase the<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">memory content on soft reboot, which gives access to the shm-path buffers after crash/reboot.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">We don't usually use /dev/shm as an argument to --shm-path. This path is normally used to hold POSIX shared memory<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">files. I would expect a different mount point path as argument to --shm-path which points to a DAX-enabled filesystem<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">backed by a PMEM block device.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">Thanks,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">Mathieu<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">--
<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">Mathieu Desnoyers<br>
EfficiOS Inc.<br>
<a href="https://clicktime.symantec.com/32vnSPUsVCfHCkkwoyPymVx7Vc?u=http%3A%2F%2Fwww.efficios.com">https://clicktime.symantec.com/32vnSPUsVCfHCkkwoyPymVx7Vc?u=http%3A%2F%2Fwww.efficios.com</a><o:p></o:p></span></p>
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