Messages in this thread | | | Date | Thu, 19 Feb 2015 15:42:29 -0600 | From | Josh Poimboeuf <> | Subject | Re: [PATCH 1/3] sched: add sched_task_call() |
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On Thu, Feb 19, 2015 at 09:40:36PM +0100, Vojtech Pavlik wrote: > On Thu, Feb 19, 2015 at 11:32:55AM -0600, Josh Poimboeuf wrote: > > On Thu, Feb 19, 2015 at 06:19:29PM +0100, Vojtech Pavlik wrote: > > > On Thu, Feb 19, 2015 at 11:03:53AM -0600, Josh Poimboeuf wrote: > > > > On Thu, Feb 19, 2015 at 05:33:59PM +0100, Vojtech Pavlik wrote: > > > > > On Thu, Feb 19, 2015 at 10:24:29AM -0600, Josh Poimboeuf wrote: > > > > > > > > > > > > No, these tasks will _never_ make syscalls. So you need to guarantee > > > > > > > they don't accidentally enter the kernel while you flip them. Something > > > > > > > like so should do. > > > > > > > > > > > > > > You set TIF_ENTER_WAIT on them, check they're still in userspace, flip > > > > > > > them then clear TIF_ENTER_WAIT. > > > > > > > > > > > > Ah, that's a good idea. But how do we check if they're in user space? > > > > > > > > > > I don't see the benefit in holding them in a loop - you can just as well > > > > > flip them from the syscall code as kGraft does. > > > > > > > > But we were talking specifically about HPC tasks which never make > > > > syscalls. > > > > > > Yes. I'm saying that rather than guaranteeing they don't enter the > > > kernel (by having them spin) you can flip them in case they try to do > > > that instead. That solves the race condition just as well. > > > > Ok, gotcha. > > > > We'd still need a safe way to check if they're in user space though. > > Having a safe way would be very nice and actually quite useful in other > cases, too. > > For this specific purpose, however, we don't need a very safe way, > though. We don't require atomicity in any way, we don't mind even if it > creates false negatives, only false positives would be bad. > > kGraft looks at the stacktrace of CPU hogs and if it finds no kernel > addresses there, it assumes userspace. Not very nice, but does the job.
So I've looked at kgr_needs_lazy_migration(), but I still have no idea how it works.
First of all, I think reading the stack while its being written to could give you some garbage values, and a completely wrong nr_entries value from save_stack_trace_tsk().
But also, how would you walk a stack without knowing its stack pointer? That function relies on the saved stack pointer in task_struct.thread.sp, which, AFAICT, was last saved during the last call to schedule(). Since then, the stack could have been completely rewritten, with different size stack frames, before the task exited the kernel.
Am I missing something?
-- Josh
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