Messages in this thread | | | Date | Mon, 19 May 2014 13:43:32 +0530 | From | Preeti U Murthy <> | Subject | Re: [PATCH] sched: fix exec_start/task_hot on migrated tasks |
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On 05/16/2014 07:47 PM, Peter Zijlstra wrote: > 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. >
Ok now I understand this.Thanks!
Reviewed-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>
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