On Fri, Jun 17, 2011 at 01:02:47AM +0200, Ingo Molnar wrote: > > * Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote: > > > On Thu, Jun 16, 2011 at 10:25:50PM +0200, Ingo Molnar wrote: > > > > > > * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > The funny thing about this workload is that context-switches are > > > > > really a fastpath here and we are using anonymous IRQ-triggered > > > > > softirqs embedded in random task contexts as a workaround for > > > > > that. > > > > > > > > The other thing that the IRQ-triggered softirqs do is to get the > > > > callbacks invoked in cases where a CPU-bound user thread is never > > > > context switching. > > > > > > Yeah - but this workload didnt have that. > > > > > > > Of course, one alternative might be to set_need_resched() to force > > > > entry into the scheduler as needed. > > > > > > No need for that: we can just do the callback not in softirq but in > > > regular syscall context in that case, in the return-to-userspace > > > notifier. (see TIF_USER_RETURN_NOTIFY and the USER_RETURN_NOTIFIER > > > facility) > > > > > > Abusing a facility like setting need_resched artificially will > > > generally cause trouble. > > > > If the task enqueued callbacks in the kernel, thus started a new > > grace period, it might return to userspace before every CPUs have > > completed that grace period, and you need that full completion to > > happen before invoking the callbacks. > > > > I think you need to keep the tick in such case because you can't > > count on the other CPUs to handle that completion as they may be > > all idle. > > > > So when you resume to userspace and you started a GP, either you > > find another CPU to handle the GP completion and callbacks > > executions, or you keep the tick until you are done. > > We'll have a scheduler tick in any case, which will act as a > worst-case RCU tick. > > My main point is that we need to check whether this solution improves > performance over the current softirq code. I think there's a real > chance that it improves things like VFS workloads, because it > provides (much!) lower grace period latencies hence provides > fundamentally better cache locality. > > If a workload pays the cost of frequent scheduling then it might as > well use a beneficial side-effect of that scheduling: high-freq grace > periods ... > > If it improves performance we can figure out all the loose ends. If > it doesnt then the loose ends are not worth worrying about. Yeah I see your point, seems worth trying. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>