* 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.) Also, no matter what scheme we use, the target CPU does have to be processed somehow and it does have to signal completion back somehow - which generates cachemisses. I think what probaby matters most is to go simple, and to use established kernel primitives - and the above is really typical pattern for things like TLB flushes to a process having a presence on every physical CPU. Those aspects will be kept reasonably fast and balanced on all hardware that matters. (and if not, people will notice any TLB flush/shootdown linear slowdowns and will address it) I could be wrong though ... maybe someone can get some numbers from a really large system? 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
