Hi,
On 3/14/25 2:08 PM, Benjamin Berg wrote:
From: Benjamin Berg benjamin.berg@intel.com um: work around sched_yield not yielding in time-travel mode
sched_yield by a userspace may not actually cause scheduling in time-travel mode as no time has passed. In the case seen it appears to be a badly implemented userspace spinlock in ASAN. Unfortunately, with time-travel it causes an extreme slowdown or even deadlock depending on the kernel configuration (CONFIG_UML_MAX_USERSPACE_ITERATIONS).
Work around it by accounting time to the process whenever it executes a sched_yield syscall.
Signed-off-by: Benjamin Berg benjamin.berg@intel.com
From what I can tell the patch mentioned above was backported to 6.12.27 by: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/arch/um?id=887c5c12e80c8424bd471122d2e8b6b462e12874
but without the upstream |Commit 0b8b2668f9981c1fefc2ef892bd915288ef01f33 |Author: Benjamin Berg benjamin.berg@intel.com |Date: Thu Oct 10 16:25:37 2024 +0200 | um: insert scheduler ticks when userspace does not yield | | In time-travel mode userspace can do a lot of work without any time | passing. Unfortunately, this can result in OOM situations as the RCU | core code will never be run. [...]
the kernel build for 6.12.27 for the UM-Target will fail:
| /usr/bin/ld: arch/um/kernel/skas/syscall.o: in function `handle_syscall': linux-6.12.27/arch/um/kernel/skas/syscall.c:43:(.text+0xa2): undefined reference to `tt_extra_sched_jiffies' | collect2: error: ld returned 1 exit status
is it possible to backport 0b8b2668f9981c1fefc2ef892bd915288ef01f33 too? Or is it better to revert 887c5c12e80c8424bd471122d2e8b6b462e12874 again in the stable releases?
Best Regards, Christian Lamparter
I suspect it is this code in ASAN that uses sched_yield https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/sanitizer_com... though there are also some other places that use sched_yield.
I doubt that code is reasonable. At the same time, not sure that sched_yield is behaving as advertised either as it obviously is not necessarily relinquishing the CPU.
arch/um/include/linux/time-internal.h | 2 ++ arch/um/kernel/skas/syscall.c | 11 +++++++++++ 2 files changed, 13 insertions(+)
diff --git a/arch/um/include/linux/time-internal.h b/arch/um/include/linux/time-internal.h index b22226634ff6..138908b999d7 100644 --- a/arch/um/include/linux/time-internal.h +++ b/arch/um/include/linux/time-internal.h @@ -83,6 +83,8 @@ extern void time_travel_not_configured(void); #define time_travel_del_event(...) time_travel_not_configured() #endif /* CONFIG_UML_TIME_TRAVEL_SUPPORT */ +extern unsigned long tt_extra_sched_jiffies;
- /*
- Without CONFIG_UML_TIME_TRAVEL_SUPPORT this is a linker error if used,
- which is intentional since we really shouldn't link it in that case.
diff --git a/arch/um/kernel/skas/syscall.c b/arch/um/kernel/skas/syscall.c index b09e85279d2b..a5beaea2967e 100644 --- a/arch/um/kernel/skas/syscall.c +++ b/arch/um/kernel/skas/syscall.c @@ -31,6 +31,17 @@ void handle_syscall(struct uml_pt_regs *r) goto out; syscall = UPT_SYSCALL_NR(r);
- /*
* If no time passes, then sched_yield may not actually yield, causing* broken spinlock implementations in userspace (ASAN) to hang for long* periods of time.*/- if ((time_travel_mode == TT_MODE_INFCPU ||
time_travel_mode == TT_MODE_EXTERNAL) &&syscall == __NR_sched_yield)tt_extra_sched_jiffies += 1;- if (syscall >= 0 && syscall < __NR_syscalls) { unsigned long ret = EXECUTE_SYSCALL(syscall, regs);