On Thu, Oct 29, 2015 at 04:30:20AM +0000, Hiroshi Shimamoto wrote: > > Subject: Re: [PATCH v2] cputime: fix invalid gtime > > > > On Thu, Oct 29, 2015 at 01:10:01AM +0000, Hiroshi Shimamoto wrote: > > > > Obviously I completely messed up there. And task_cputime() has a > > > > similar issue > > > > but it happens to work due to vtime_snap_whence set to VTIME_SLEEPING > > > > when vtime > > > > doesn't run. Still it works at the cost of a seqcount read operation. > > > > > > > > Do you think you could fix it too (along with task_cputime_scaled())? I > > > > think those > > > > patches will also need a stable tag. > > > > > > Do you mean that task_cputime() and task_cputime_scaled() don't hit > > > invalid behavior > > > but have some extra operation cost which could be removed? > > > > Exactly. > > > > > > > > Will look into it, and send patches with stable tag. > > > > Thanks a lot! > > > > Oh and another detail: vtime_accounting_enabled() checks if vtime > > accounting is done precisely on the current CPU. That's what we want to > > check > > when we account the time but not when we want to read the cputime of a task. > > > > For example, CPU 0 never has vtime_accounting_enabled() because it plays the > > role of timekeeper and as such it keeps the tick periodic. So if task A > > runs on > > CPU 1 that has vtime accounting on, and we read the cputime of task A from > > CPU 0, > > vtime_accounting_enabled() will be false whereas we need to compute the > > delta. > > > > So vtime_accounting_enabled() isn't suitable to check if vtime is running > > on _some_ > > CPU such that we can't return utime/stime with a raw read. > > I see the point, vtime accounting can be enabled on dedicated cpu and there > is no > guarantee the reading thread is on the same state.
Exactly! > > > > > Ideally we shoud rename vtime_accounting_enabled() to > > vtime_accounting_cpu_enabled() > > and have vtime_accounting_enabled() to check if vtime runs somewhere. But > > that would > > be too much an invasive change for a stable patch. So lets just use > > context_tracking_is_enabled() for now instead. > > I have dig the code. > And my understanding is that vtime_accounting_enabled() does check global > flag with > context_tracking_is_enabled() and then check current cpu state with > context_tracking_cpu_is_enabled(). For now, we just check global flag to fix > current > issue instead of checking both in vtime_accounting_enabled(). In future we > should fix > more precisely. > Is that correct? Perfectly correct :-) Thanks! -- 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/
