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gem5 syscall: better describe behaviour

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Ciro Santilli 六四事件 法轮功
2020-02-21 00:00:00 +00:00
parent e644cc6eb3
commit ed4be5fd89
1 changed files with 13 additions and 17 deletions
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README.adoc
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@@ -10820,15 +10820,17 @@ At 369a47fc6e5c2f4a7f911c1c058b6088f8824463 + 1 QEMU appears to spawn 3 host thr
====== gem5 syscall emulation multithreading
gem5 user mode multithreading has been particularly flaky compared <<qemu-user-mode-multithreading,to QEMU's>>.
gem5 user mode multithreading has been particularly flaky compared <<qemu-user-mode-multithreading,to QEMU's>>, but work is being put into improving it.
You have the limitation that you must have at least one core per guest thread, otherwise `pthread_create` fails. For example:
In gem5 syscall simulation, the `fork` syscall checks if there is a free CPU, and if there is a free one, the new threads runs on that CPU. Otherwise, the `fork` call, and therefore higher level interfaces to `fork` such as `pthread_create` also fail and return a failure return status in the guest.
For example, if we use just one CPU for link:userland/posix/pthread_self.c[] which spawns one thread besides `main`:
....
./run --cpus 1 --emulator gem5 --userland userland/posix/pthread_self.c --userland-args 1
....
fails because that process has a total of 2 threads: one for `main` and one extra thread spawned: link:userland/posix/pthread_self.c[] The error message is:
fails with this error message coming from the guest stderr:
....
pthread_create: Resource temporarily unavailable
@@ -10840,9 +10842,15 @@ It works however if we add on extra CPU:
./run --cpus 2 --emulator gem5 --userland userland/posix/pthread_self.c --userland-args 1
....
This has to do with the fact that gem5 has a more simplistic thread implementation that does not spawn one host thread per guest thread CPU. Maybe this is required to achieve reproducible runs? What is the task switch algorithm then?
Once threads exit, their CPU is freed and becomes available for new `fork` calls: For example, the following run spawns a thread, joins it, and then spawns again, and 2 CPUs are enough:
gem5 threading does however show the expected number of cores, e.g.:
....
./run --cpus 2 --emulator gem5 --userland userland/posix/pthread_self.c --userland-args '1 2'
....
because at each point in time, only up to two threads are running.
gem5 syscall emulation does show the expected number of cores when queried, e.g.:
....
./run --cpus 1 --userland userland/cpp/thread_hardware_concurrency.cpp --emulator gem5
@@ -10851,18 +10859,6 @@ gem5 threading does however show the expected number of cores, e.g.:
outputs `1` and `2` respectively.
TODO: aarch64 seems to failing to spawn more than 2 threads at 369a47fc6e5c2f4a7f911c1c058b6088f8824463 + 1:
....
./run --arch aarch64 --cpus 3 --emulator gem5 --userland userland/posix/pthread_self.c --userland-args 2
....
fails with:
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Exiting @ tick 18446744073709551615 because simulate() limit reached
....
====== gem5 se.py user mode with 2 or more pthreads fails with because simulate() limit reached
See bug report at: https://github.com/cirosantilli/linux-kernel-module-cheat/issues/81