On Thu, Dec 17, 2020 at 12:55:39PM -0800, Dave Hansen wrote: > On 11/6/20 3:29 PM, ira.weiny@xxxxxxxxx wrote: > > + /* Arm for context switch test */ > > + write(fd, "1", 1); > > + > > + /* Context switch out... */ > > + sleep(4); > > + > > + /* Check msr restored */ > > + write(fd, "2", 1); > > These are always tricky. What you ideally want here is: > > 1. Switch away from this task to a non-PKS task, or > 2. Switch from this task to a PKS-using task, but one which has a > different PKS value Or both... > > then, switch back to this task and make sure PKS maintained its value. > > *But*, there's no absolute guarantee that another task will run. It > would not be totally unreasonable to have the kernel just sit in a loop > without context switching here if no other tasks can run. > > The only way you *know* there is a context switch is by having two tasks > bound to the same logical CPU and make sure they run one after another. Ah... We do that. ... + CPU_ZERO(&cpuset); + CPU_SET(0, &cpuset); + /* Two processes run on CPU 0 so that they go through context switch. */ + sched_setaffinity(getpid(), sizeof(cpu_set_t), &cpuset); ... I think this should be ensuring that both the parent and the child are running on CPU 0. At least according to the man page they should be. <man> A child created via fork(2) inherits its parent's CPU affinity mask. </man> Perhaps a better method would be to synchronize the 2 threads more to ensure that we are really running at the 'same time' and forcing the context switch. > This just gets itself into a state where it *CAN* context switch and > prays that one will happen. Not sure what you mean by 'This'? Do you mean that running on the same CPU will sometimes not force a context switch? Or do you mean that the sleeps could be badly timed and the 2 threads could run 1 after the other on the same CPU? The latter is AFAICT the most likely case. > > You can also run a bunch of these in parallel bound to a single CPU. > That would also give you higher levels of assurance that *some* context > switch happens at sleep(). I think more cycles is a good idea for sure. But I'm more comfortable with forcing the test to be more synchronized so that it is actually running in the order we think/want it to be. > > One critical thing with these tests is to sabotage the kernel and then > run them and make *sure* they fail. Basically, if you screw up, do they > actually work to catch it? I'll try and come up with a more stressful test. Ira
