Messages in this thread | | | From | Rasmus Villemoes <> | Subject | Re: [RFC] [PATCH] Pre-emption control for userspace | Date | Thu, 06 Mar 2014 14:24:43 +0100 |
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"H. Peter Anvin" <hpa@zytor.com> writes:
> I have several issues with this interface: > > 1. First, a process needs to know if it *should* have been preempted > before it calls sched_yield(). So there needs to be a second flag set > by the scheduler when granting amnesty. > > 2. A process which fails to call sched_yield() after being granted > amnesty must be penalized. > > 3. I'm not keen on occupying a full page for this. I'm wondering if > doing a pointer into user space, futex-style, might make more sense. > The downside, of course, is what happens if the page being pointed to is > swapped out.
Is it possible to implement non-sleeping versions of {get,put}_user()? That is, use the same basic approach (let the MMU do the hard work) but use different, and simpler, "fixup" code (return -ESOMETHING on a major fault).
If so, I think an extra pointer (->amnesty) and an extra bit (->amnesty_granted) in task_struct suffices:
If the scheduler runs and tsk->amnesty_granted is true, penalize the current task (and possibly clear tsk->amnesty_granted).
Otherwise, the task is granted amnesty provided these conditions are met:
tsk->amnesty is non-NULL get_user_nosleep(j, tsk->amnesty) succeeds j is now 1 put_user_nosleep(YOU_WERE_LUCKY, tsk->amnesty) succeeds
If so, set amnesty_granted and let the thread continue; otherwise reschedule.
The userspace side would be something like
void *thread_func(void*) { int Im_busy = 0; sched_need_amnesty(&Im_busy); /* better name needed */ /* -EPERM if not allowed (new capability?) */
/* go critical */ Im_busy = 1; LOCK(); do_stuff(); UNLOCK(); if (Im_busy != 1) { /* better play nice with the others */ sched_yield(); } Im_busy = 0;
/* If the thread doesn't need the amnesty feature anymore, it can just do sched_need_amnesty(NULL); */ }
Rasmus
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