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9a6ecbd590
The thread dump mechanism that is used by "show threads" and by the panic dump is overly complicated due to an initial misdesign. It firsts wakes all threads, then serializes their dumps, then releases them, while taking extreme care not to face colliding dumps. In fact this is not what we need and it reached a limit where big machines cannot dump all their threads anymore due to buffer size limitations. What is needed instead is to be able to dump *one* thread, and to let the requester iterate on all threads. That's what this patch does. It adds the thread_dump_buffer to the struct thread_ctx so that the requester offers the buffer to the thread that is about to be dumped. This buffer also serves as a lock. A thread at rest has a NULL, a valid pointer indicates the thread is using it, and 0x1 (NULL+1) is used by the dumped thread to tell the requester it's done. This makes sure that a given thread is dumped once at a time. In addition to this, the calling thread decides whether it accesses the thread by itself or via the debug signal handler, in order to get a backtrace. This is much saner because the calling thread is free to do whatever it wants with the buffer after each thread is dumped, and there is no dependency between threads, once they've dumped, they're free to continue (and possibly to dump for another requester if needed). Finally, when the THREAD_DUMP feature is disabled and the debug signal is not used, the requester accesses the thread by itself like before. For now we still have the buffer size limitation but it will be addressed in future patches. |
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