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Message-ID: <87ob1jbgtg.fsf@rasmusvillemoes.dk> Date: Thu, 06 Mar 2014 14:24:43 +0100 From: Rasmus Villemoes <linux@...musvillemoes.dk> To: "H. Peter Anvin" <hpa@...or.com> Cc: Khalid Aziz <khalid.aziz@...cle.com>, tglx@...utronix.de, Ingo Molnar <mingo@...nel.org>, peterz@...radead.org, akpm@...ux-foundation.org, andi.kleen@...el.com, rob@...dley.net, viro@...iv.linux.org.uk, oleg@...hat.com, venki@...gle.com, linux-kernel@...r.kernel.org Subject: Re: [RFC] [PATCH] Pre-emption control for userspace "H. Peter Anvin" <hpa@...or.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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