On Fri, May 16, 2014 at 07:27:32PM +0530, Preeti Murthy wrote: > > 0 isn't strictly the right thing to do here, since the clock can wrap, > > but being wrong every ~585 years isn't too big an issue for this. > > I don't understand this. Will setting it to 0 not indicate beginning > of ticking?
In modular spaces there is no beginnings nor endings. > So when you find out for how long the task has run, the > difference would be larger than what would have been had you > let exec_start be at its previous value of the old cpu's clock_task right? Nope, see the modular space thing, once every ~585 years we'll wrap the clock and 0 is within range of recently ran. > Will not setting exec_start to the clock_task of the destination rq > during migration be better? This would be the closest we could > come to estimating the amount of time the task has run on this new > cpu while deciding task_hot or not no? Setting it to the exact clock_task of the destination rq would make it hot on that rq, even though it hasn't yet ran there, so you'd have to do something like: rq_clock_task(dst_rq) - sysctl_sched_migration_cost. But seeing as how that is far more work, and all this is heuristics anyhow and an extra fail term of 1/585 years is far below the current fail rate, all is well.
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