On Thu, Dec 17, 2020 at 8:05 PM Ira Weiny ira.weiny@intel.com wrote:
On Thu, Dec 17, 2020 at 12:55:39PM -0800, Dave Hansen wrote:
On 11/6/20 3:29 PM, ira.weiny@intel.com 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:
- Switch away from this task to a non-PKS task, or
- 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.
One way to guarantee that both threads run is to just pass a message between them over a pipe and wait for the submitter to receive an ack from the other end.