* Avi Kivity <avi@xxxxxxxxxx> wrote: > On 05/29/2011 10:35 AM, Ingo Molnar wrote: > >* Avi Kivity<avi@xxxxxxxxxx> wrote: > > > >> On 05/28/2011 09:32 PM, Ingo Molnar wrote: > >> >* Avi Kivity<avi@xxxxxxxxxx> wrote: > >> > > >> >> > So if you set a notification signal via fcntl(F_SETOWN) on the > >> >> > scheduler context switch event fd, the user-space RCU code will > >> >> > get a signal on every context switch. > >> >> > >> >> Context switches are completely uninteresting for userspace rcu: > >> >> > >> >> rcu_read_lock(); > >> >> ---> context switch > >> >> > >> >> have we learned anything from that? no. User code is always > >> >> preemptible and migratable. If rcu_read_lock() prevented migration > >> >> somehow, then we'd know that a context switch means we've started a > >> >> grace period for this thread. But it doesn't, so we don't. > >> > > >> >Well, in the next mail i mentioned that we can do migration events as > >> >well, which would be useful: instead of having to keep track of > >> >nr_tasks RCU grace periods we could simplify it down to nr_cpus. > >> > >> I don't see how a migration event helps. It is completely > >> transparent from the task's point of view. > > > > It's not transparent at all if you index RCU data structures by > > the current CPU index, which the kernel implementation does. > > But that's completely broken for userspace. The "current cpu > index" doesn't even exist, since you can't disable preemption. It does exist, if the signal handler notification of a migration is instantaneous (which it is). > > Doing that has the advantage of being much more cache-compressed > > than the TID index, > > If you have more tasks than cpus; which isn't a given. Sure there are special cases but in general there can be many more tasks (threads) than CPUs ;-) Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
