* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > On Wed, May 20, 2009 at 10:09:24AM +0200, Ingo Molnar wrote: > > > > * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > > On Tue, May 19, 2009 at 02:44:36PM +0200, Ingo Molnar wrote: > > > > > > > > * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > On Tue, May 19, 2009 at 10:58:25AM +0200, Ingo Molnar wrote: > > > > > > > > > > > > * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > > On Mon, May 18, 2009 at 05:42:41PM +0200, Ingo Molnar wrote: > > > > > > > > > > > > > > > > * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > i might be missing something fundamental here, but why not just > > > > > > > > > > have per CPU helper threads, all on the same waitqueue, and wake > > > > > > > > > > them up via a single wake_up() call? That would remove the SMP > > > > > > > > > > cross call (wakeups do immediate cross-calls already). > > > > > > > > > > > > > > > > > > My concern with this is that the cache misses accessing all the > > > > > > > > > processes on this single waitqueue would be serialized, slowing > > > > > > > > > things down. In contrast, the bitmask that smp_call_function() > > > > > > > > > traverses delivers on the order of a thousand CPUs' worth of bits > > > > > > > > > per cache miss. I will give it a try, though. > > > > > > > > > > > > > > > > At least if you go via the migration threads, you can queue up > > > > > > > > requests to them locally. But there's going to be cachemisses > > > > > > > > _anyway_, since you have to access them all from a single CPU, > > > > > > > > and then they have to fetch details about what to do, and then > > > > > > > > have to notify the originator about completion. > > > > > > > > > > > > > > Ah, so you are suggesting that I use smp_call_function() to run > > > > > > > code on each CPU that wakes up that CPU's migration thread? I > > > > > > > will take a look at this. > > > > > > > > > > > > My suggestion was to queue up a dummy 'struct migration_req' up with > > > > > > it (change migration_req::task == NULL to mean 'nothing') and simply > > > > > > wake it up using wake_up_process(). > > > > > > > > > > OK. I was thinking of just using wake_up_process() without the > > > > > migration_req structure, and unconditionally setting a per-CPU > > > > > variable from within migration_thread() just before the list_empty() > > > > > check. In your approach we would need a NULL-pointer check just > > > > > before the call to __migrate_task(). > > > > > > > > > > > That will force a quiescent state, without the need for any extra > > > > > > information, right? > > > > > > > > > > Yep! > > > > > > > > > > > This is what the scheduler code does, roughly: > > > > > > > > > > > > wake_up_process(rq->migration_thread); > > > > > > wait_for_completion(&req.done); > > > > > > > > > > > > and this will always have to perform well. The 'req' could be put > > > > > > into PER_CPU, and a loop could be done like this: > > > > > > > > > > > > for_each_online_cpu(cpu) > > > > > > wake_up_process(cpu_rq(cpu)->migration_thread); > > > > > > > > > > > > for_each_online_cpu(cpu) > > > > > > wait_for_completion(&per_cpu(req, cpu).done); > > > > > > > > > > > > hm? > > > > > > > > > > My concern is the linear slowdown for large systems, but this > > > > > should be OK for modest systems (a few 10s of CPUs). However, I > > > > > will try it out -- it does not need to be a long-term solution, > > > > > after all. > > > > > > > > I think there is going to be a linear slowdown no matter what - > > > > because sending that many IPIs is going to be linear. (there are > > > > no 'broadcast to all' IPIs anymore - on x86 we only have them if > > > > all physical APIC IDs are 7 or smaller.) > > > > > > With the current code, agreed. One could imagine making an IPI > > > tree, so that a given CPU IPIs (say) eight subordinates. Making > > > this work nice with CPU hotplug would be entertaining, to say the > > > least. > > > > Certainly! :-) > > > > As a general note, unrelated to your patches: i think our > > CPU-hotplug related complexity seems to be a bit too much. This is > > really just a gut feeling - from having seen many patches that also > > have hotplug notifiers. > > > > I'm wondering whether this is because it's structured in a > > suboptimal way, or because i'm (intuitively) under-estimating the > > complexity of what it takes to express what happens when a CPU is > > offlined and then onlined? > > I suppose that I could take this as a cue to reminisce about the > old days in a past life with a different implementation of CPU > online/offline, but life is just too short for that sort of thing. > Not that guys my age let that stop them. ;-) > > And in that past life, exercising CPU online/offline usually > exposed painful bugs in new code, so I cannot claim that the > old-life approach to CPU hotplug was perfect. Interestingly > enough, running uniprocessor also exposed painful bugs more often > than not. Of course, the only way to run uniprocessor was to > offline all but one of the CPUs, so you would hit the > online/offline bugs before hitting the uniprocessor-only bugs. > > The thing that worries me most about CPU hotplug in Linux is that > there is no clear hierarchy of CPU function in the offline > process, given that the offlining process invokes notifiers in the > same order as does the onlining process. Whether this is a real > defect in the CPU hotplug design or is instead simply a symptom of > my not yet being fully comfortable with the two-phase CPU-removal > process is an interesting question to which I do not have an > answer. I strongly believe it's the former. > Either way, the thought process is different. In my old life, > CPUs shed roles in the opposite order that they acquired them. Yeah, that looks a whole lot more logical to do. > This meant that a given CPU was naturally guaranteed to be > correctly taking interrupts for the entire time that it was > capable of running user-level processes. Later in the offlining > process, it would still take interrupts, but would be unable to > run user processes. Still later, it would no longer be taking > interrupts, and would stop participating in RCU and in the global > TLB-flush algorithm. There was no need to stop the whole machine > to make a given CPU go offline, in fact, most of the work was done > by the CPU in question. > > In the case of RCU, this meant that there was no need for > double-checking for offlined CPUs, because CPUs could reliably > indicate a quiescent state on their way out. > > On the other hand, there was no equivalent of dynticks in the old > days. And it is dynticks that is responsible for most of the > complexity present in force_quiescent_state(), not CPU hotplug. > > So I cannot hold up RCU as something that would be greatly > simplified by changing the CPU hotplug design, much as I might > like to. ;-) We could probably remove a fair bit of dynticks complexity by removing non-dynticks and removing non-hrtimer. People could still force a 'periodic' interrupting mode (if they want, or if their hw forces that), but that would be a plain periodic hrtimer firing off all the time. Ingo -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
