Hello Peter, On Mon, 2010-11-15 at 18:21 +0100, Peter Zijlstra wrote: > On Mon, 2010-11-15 at 18:09 +0100, Michael Holzheu wrote: > > > > That you should not use sched_clock(), > > > > What should we use instead? > > Depends on what you want, look at kernel/sched_clock.c > > > > What does last departed mean? That is what timeline are you counting in? > > > Do you want time as tasks see it, or time as your wallclock sees it? > > > > "last_depart" should be the time stamp, where the task has left a CPU > > the last time. > > > > We assume that we can compare "last_depart" with "time_ns" in the > > taskstats structure, > > I think you assume I actually know anything about taskstat :-), its the > thing I always say =n to in my config file and have so far happily > ignored all code of. > > > if we use task_rq(t)->clock for last_depart and > > sched_clock() for stats->time_ns. > > Then you're up shit creek because rq->clock doesn't necessarily have any > correlation to sched_clock(). > > > We also assume that we get wallclock > > intervals in nanoseconds, if we look at two sched_clock() timestamps. > > Invalid assumption. Ok, thanks. So sched_clock() seems to be a bad idea for our purposes. An alternative approach could be to have a global counter for the task snapshots, which is increased each time a snapshot is created for userspace. In addition to that we had to add a snapshot counter field to the task_struct that is set to the current value of the global counter each time a task leaves a CPU. Then userspace could ask for all tasks that have been active after snapshot number x. In the response userspace gets all tasks that have a snapshot number bigger than x together with the new snapshot number y that can be used for the next query. Still it would be useful to add a timestamp of the creation of the taskstats data in the response to userspace for calculating the interval time between two snapshots. Would the usage of ktime_get() be valid for that purpose? Michael -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
