sched_setattr: Function not implemented

[Ox24]

Hey there,
wanted to test some realtime scheduling and tried to measure some timings and overhead caused by gvisor.

Using the debug flag on gvisor, i noticed that i get:

write(0x4, 0x7facec000af0 "sched_setattr: Function not implemented\n", 0x28)

therefore i cannot run and test my application. Any advices?

docker command : docker run --runtime=runsc --cap-add SYS_NICE -i -d -t simple-interpolation
docker version: Docker version 18.03.1-ce
uname -a : 4.9.84-rt62 SMP PREEMPT RT Tue May 8 18:11:18 CEST 2018 x86_64 x86_64 x86_64 GNU/Linux

[fvoznika]

gVisor doesn't implement sched_setattr. gVisor uses the Go runtime for scheduling, it doesn't support realtime scheduling nor priorities.

Your options are to change the application to not make sched_setattr(2) calls or to ignore errors from it.

[Ox24]

sadly that is not an option for me. We are running different rt applications on our system using docker and wanted to isolate some of them. Therefore i thougt, i could measure the impact of gvisor on rt applications. Do you guys have a in-depth descritption on how gvisor works?

[fvoznika]

You can try to stub out the required syscalls and pretend they succeeded to see how far you can get. [1] has a patch making sched_setattr a noop as an example.

We're improving the documentation as we find time. You can find more information about scheduling in gvisor/pkg/sentry/kernel/README.md

[1]

diff --git a/pkg/sentry/syscalls/linux/linux64.go b/pkg/sentry/syscalls/linux/linux64.go
index edfcdca..e06253a 100644
--- a/pkg/sentry/syscalls/linux/linux64.go
+++ b/pkg/sentry/syscalls/linux/linux64.go
@@ -186,7 +186,7 @@ var AMD64 = &kernel.SyscallTable{
                //     139: Sysfs, TODO
                140: Getpriority,
                141: Setpriority,
-               142: syscalls.CapError(linux.CAP_SYS_NICE), // SchedSetparam, requires cap_sys_nice
+               142: syscalls.Noop(),
                143: SchedGetparam,
                144: SchedSetscheduler,
                145: SchedGetscheduler,
diff --git a/pkg/sentry/syscalls/syscalls.go b/pkg/sentry/syscalls/syscalls.go
index 1176f85..9f1f25e 100644
--- a/pkg/sentry/syscalls/syscalls.go
+++ b/pkg/sentry/syscalls/syscalls.go
@@ -33,6 +33,12 @@ import (
        "gvisor.googlesource.com/gvisor/pkg/syserror"
 )
 
+func Noop() kernel.SyscallFn {
+       return func(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
+               return 0, nil, nil
+       }
+}
+
 // Error returns a syscall handler that will always give the passed error.
 func Error(err error) kernel.SyscallFn {
        return func(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {

[ianlewis]

I suggest changing the bug title to "sched_setattr: Function not implemented"

[fvoznika]

Closing as this is something unlikely to be supported.

[ianlewis]

Just for completeness, the reason why it's not likely to be supported is because we need Go runtime support for scheduling.