Hi David,
On 06/21/2017 12:12 PM, David Werner wrote:
Hi everyone,
As I'm stuck with this problem I would appreciate any kind of advice.
sorry for the long delay. See below for my comments.
Best Regards, David
Am 07.06.2017 um 15:13 schrieb David Werner:
Hi everyone,
after Denis Huber left the project, I am in charge of making our checkpoint/restore component work. Therefore i would like to ask some more questions on the IRQ kernel object.
- When is the IRQ object created? Does every component have an own
IRQ object?
It is created when the signal source client (separate thread) is created. The signal source client is created once while bootstrapping a component. It receives pure signals from the corresponding core service and delivers the signals locally, e.g.: unblocks an entrypoint that is waiting for signals and IPC.
I tried to figure out when the IRQ object is mapped into the object space of a component on its startup. Therefore I took a look at the code in [repos/base-foc/src/core/signal_source_component.cc]. The IRQ object appears in the object space after the "_sem = <Rpc_request_semaphore>();" statement in the constructor.
As far as I could follow the implementation the "request_semaphore" RPC call is answered by the "Signal_source_rpc_object" in [base-foc/src/include/signal_source/rpc_object.h] which returns/delegates the native capability "_blocking_semaphore" which is an attribute of the "Signal_source_rpc_object". It seems to me that the IRQ object already exists at this point and is only delegated to the component.
But when is the IRQ object created and by whom? Is it created when a new PD session is created?
It is created by core, when a new SIGNAL session is opened to it. This is typically done during the startup of a new component. You are right, the request_semaphore() call then just transfers the IRQ object's capability from core to the requesting component.
- Does the IRQ object carry any information? Do I need to checkpoint
this information in order to be able to recreate the object properly during a restore process? Is the IRQ object created automatically (and i only have to make sure that the object is getting mapped into the object space of the target) or do i have to create it manually?
The IRQ object does not carry information, but its state changes when a thread attaches or detaches from it. So if you re-create that specific IRQ object, the signal handler thread that is using the signal source client has to attach again to the replaced IRQ object.
In our current implementation of the restore process we restore a component by recreating its sessions to core services (+timer) with the help of information we gathered using a custom runtime environment. After the sessions are restored we place them in the object space at the correct position. Will I also have to somehow store information about the IRQ object? Or is it just some object that needs to exist?
As being said, this specific IRQ object is part of the SIGNAL session and its client state. I'm not sure how your restore mechanism works exactly, but if work is done within the component that gets restored, you can do the re-attachment there. Otherwise, you would need to change the request_semaphore call. So that the information of which thread gets attached is part of the server side in core. Instead of attaching to the IRQ object itself, the signal handler thread transfers its identity to core via request_semaphore. Core attaches the thread and delivers the capability. Whenever request_semaphore is called, you detach formerly attached threads, as well as when the session is closed. Does that make sense for you?
Regards Stefan
Kind Regards, David
Am 29.03.2017 um 14:05 schrieb Stefan Kalkowski:
Hello Dennis,
On 03/27/2017 04:14 PM, Denis Huber wrote:
Dear Genode community,
Preliminary: We implemented a Checkpoint/Restore mechanism on basis of Genode/Fiasco.OC (Thanks to the great help of you all). We store the state of the target component by monitoring its RPC function calls which go through the parent component (= our Checkpoint/Restore component). The capability space is indirectly checkpointed through the capability map. The restoring of the state of the target is done by restoring the RPC objects used by the target component (e.g. PD session, dataspaces, region maps, etc.). The capabilities of the restored objects have to be also restored in the capability space (kernel) and in the capability map (userspace).
For restoring the target component Norman suggested the usage of the Genode::Child constructor with an invalid ROM dataspace capability which does not trigger the bootstrap mechanism. Thus, we have the full control of inserting the capabilities of the restored RPC objects into the capability space/map.
Our problem is the following: We restore the RPC objects and insert them into the capability map and then in the capability space. From the kernel point of view these capabilities are all "IPC Gates". Unfortunately, there was also an IRQ kernel object created by the bootstrap mechanism. The following table shows the kernel debugger output of the capability space of the freshly bootstraped target component:
000204 :0016e* Gate 0015f* Gate 00158* Gate 00152* Gate 000208 :00154* Gate 0017e* Gate 0017f* Gate 00179* Gate 00020c :00180* Gate 00188* Gate -- -- 000210 : -- -- 0018a* Gate 0018c* Gate 000214 :0018e* Gate 00196* Gate 00145* Gate 00144* IRQ 000218 :00198* Gate -- -- -- 00021c : -- 0019c* Gate -- --
At address 000217 you can see the IRQ kernel object. What does this object do, how can we store/monitor it, and how can it be restored? Where can we find the source code which creates this object in Genode's bootstrap code?
The IRQ kernel object you refer to is used by the "signal_handler" thread to block for signals of core's corresponding service. It is a base-foc specific internal core RPC object[1] that is used by the signal handler[2] and the related capability gets returned by the call to 'alloc_signal_source()' provided by the PD session[3].
I have to admit, I did not follow your current implementation approach in depth. Thereby, I do not know how to exactly handle this specific signal hander thread and its semaphore-like IRQ object, but maybe the references already help you further.
Regards Stefan
[1] repos/base-foc/src/core/signal_source_component.cc [2] repos/base-foc/src/lib/base/signal_source_client.cc [3] repos/base/src/core/include/pd_session_component.h
Best regards, Denis
On 11.12.2016 13:01, Denis Huber wrote:
Hello Norman,
What you observe here is the ELF loading of the child's binary. As part of the 'Child' object, the so-called '_process' member is constructed. You can find the corresponding code at 'base/src/lib/base/child_process.cc'. The code parses the ELF executable and loads the program segments, specifically the read-only text segment and the read-writable data/bss segment. For the latter, a RAM dataspace is allocated and filled with the content of the ELF binary's data. In your case, when resuming, this procedure is wrong. After all, you want to supply the checkpointed data to the new child, not the initial data provided by the ELF binary.
Fortunately, I encountered the same problem when implementing fork for noux. I solved it by letting the 'Child_process' constructor accept an invalid dataspace capability as ELF argument. This has two effects: First, the ELF loading is skipped (obviously - there is no ELF to load). And second the creation of the initial thread is skipped as well.
In short, by supplying an invalid dataspace capability as binary for the new child, you avoid all those unwanted operations. The new child will not start at 'Component::construct'. You will have to manually create and start the threads of the new child via the PD and CPU session interfaces.
Thank you for the hint. I will try out your approach
The approach looks good. I presume that you encounter base-foc-specific peculiarities of the thread-creation procedure. I would try to follow the code in 'base-foc/src/core/platform_thread.cc' to see what the interaction of core with the kernel looks like. The order of operations might be important.
One remaining problem may be that - even though you may by able the restore most part of the thread state - the kernel-internal state cannot be captured. E.g., think of a thread that was blocking in the kernel via 'l4_ipc_reply_and_wait' when checkpointed. When resumed, the new thread can naturally not be in this blocking state because the kernel's state is not part of the checkpointed state. The new thread would possibly start its execution at the instruction pointer of the syscall and issue system call again, but I am not sure what really happens in practice.
Is there a way to avoid this situation? Can I postpone the checkpoint by letting the entrypoint thread finish the intercepted RPC function call, then increment the ip of child's thread to the next command?
I think that you don't need the LOG-session quirk if you follow my suggestion to skip the ELF loading for the restored component altogether. Could you give it a try?
You are right, the LOG-session quirk seems a bit clumsy. I like your idea of skipping the ELF loading and automated creation of CPU threads more, because it gives me the control to create and start the threads from the stored ip and sp.
Best regards, Denis
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