Yuyang Du <yuyang...@intel.com> writes: > Thanks, Peter. > > On Wed, Jul 09, 2014 at 08:45:43PM +0200, Peter Zijlstra wrote: > >> Nope :-).. we got rid of that lock for a good reason. >> >> Also, this is one area where I feel performance really trumps >> correctness, we can fudge the blocked load a little. So the >> sched_clock_cpu() difference is a strict upper bound on the >> rq_clock_task() difference (and under 'normal' circumstances shouldn't >> be much off). > > Strictly, migrating wakee task on remote CPU entails two steps: > > (1) Catch up with task's queue's last_update_time, and then substract > > (2) Cache up with "current" time of remote CPU (for comparable matter), and > then > on new CPU, change to the new timing source (when enqueue) > > So I will try sched_clock_cpu(remote_cpu) for step (2). For step (2), maybe we > should not use cfs_rq_clock_task anyway, since the task is about to going > to another CPU/queue. Is this right?
So, sched_clock(_cpu) can be arbitrarily far off of cfs_rq_clock_task, so you can't really do that. Ideally, yes, you would account for any time since the last update and account that time as !runnable. However, I don't think there is any good way to do that, and the current code doesn't. > > I made another mistake. Should not only track task entity load, group entity > (as an entity) is also needed. Otherwise, task_h_load can't be done > correctly... > Sorry for the messup. But this won't make much change in the codes. This will increase it to 2x __update_load_avg per cgroup per enqueue/dequeue. What does this (and this patch in general) do to context switch cost at cgroup depth 1/2/3? > > Thanks, > Yuyang > >> So we could simply use a timestamps from dequeue and one from enqueue, >> and use that. >> >> As to the remote subtraction, a RMW on another cacheline than the >> rq->lock one should be good; esp since we don't actually observe the >> per-rq total often (once per tick or so) I think, no? >> >> The thing is, we do not want to disturb scheduling on whatever cpu the >> task last ran on if we wake it to another cpu. Taking rq->lock wrecks >> that for sure. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
