On Mon, Aug 15, 2016 at 10:58:04AM +0100, Mel Gorman wrote: > On Mon, Aug 15, 2016 at 11:19:01AM +0200, Stanislaw Gruszka wrote: > > > Is this really equivalent though? It updates one task instead of all > > > tasks in the group and there is no guarantee that tsk == current. > > > > Oh, my intention was to update runtime on current. > > > > Ok, so minimally that would need addressing. However, then I would worry > that two tasks in a group calling the function at the same time would > see different results because each of them updated a different task. > Such a situation is inherently race-prone anyway but it's a large enough > functional difference to be worth calling out.
It races bacause we don't know which thread will call the clock_gettime() first. But once that happen, second thread will see updated runtime value from first thread as we call update_curr() for it with task_rq_lock (change from commit 6e998916dfe3). > Minimally, I don't think such a patch is a replacement for Giovanni's > which is functionally equivalent to the current code but could be layered > on top if it is proven to be ok. I agree. I wanted to post my patch on top of Giovanni's. > > > Glancing at it, it should monotonically increase but it looks like it > > > would calculate stale data. > > > > Yes, until the next tick on a CPU, the patch does not count partial > > runtime of thread running on that CPU. However that was the behaviour > > before commit d670ec13178d0 - that how old thread_group_sched_runtime() > > function worked: > > > > Sure, but does this patch not reintroduce the "SMP wobble" and the > problem of "the diff of 'process' should always be >= the diff of > 'thread'" ? It should not reintroduce that problem, also because of change from commit 6e998916dfe3. When a thread reads sum_exec_runtime it also update that value, then process reads updated value. I run test case from "SMP wobble" commit and the problem do not happen on my tests. Perhaps I should post patch with a descriptive changelog and things would be clearer ... Stanislaw
