RE: [PATCH v2] cputime: fix invalid gtime

[Hiroshi Shimamoto]

> 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.

> 
> 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?

thanks,
Hiroshi

> 
> I think task_gtime() is also buggy when the target task runs in a CPU that doesn't do
> vtime accounting (whereas another CPU does vtime accounting). We should fix that with
> using a new VTIME_GUEST value instead of (or along with) PF_VCPU. But that's another story,
> your fix is much more important for now.
> 
> >
> > thanks,
> > Hiroshi
> >
> > >
> > > Thanks!
> > >
> > >
> > >
> > > >  	do {
> > > >  		seq = read_seqbegin(&t->vtime_seqlock);
> > > >
> > > > --
> > > > 1.8.3.1
> > > >
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