Re: [GIT PULL] isolation: 1Hz residual tick offloading v4

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 25 May 2018 04:56:25 +0200
Frederic Weisbecker <frederic@kernel.org> wrote:

> On Tue, May 22, 2018 at 10:10:19PM +0300, Yauheni Kaliuta wrote:
> > Hi, Frederic!
> >   
> > >>>>> On Mon, 29 Jan 2018 02:10:26 +0100, Frederic Weisbecker  wrote:  
> >  > On Wed, Jan 24, 2018 at 10:46:08AM -0500, Luiz Capitulino wrote:  
> > 
> > [...]
> >   
> >  >> Since the 1Hz tick offload worked for you, I must be missing
> >  >> a way to disable this timer or the kernel is thinking my CPU
> >  >> has unstable TSC (which it doesn't AFAIK).  
> >   
> >  > It's beyond the scope of this patchset but indeed that's
> >  > right, I run my kernels with tsc=reliable because my CPUs
> >  > don't have the TSC_RELIABLE flag.  That's the only way I found
> >  > to shutdown the tick completely on my test machine, otherwise
> >  > I keep having that clocksource watchdog.  
> > 
> > [...]
> > 
> > Thanks, it helps. But I have accounting problem:
> > 
> > if I run user busy loop on the nohz cpu, the task accounting works
> > correctly (top shows the task takes 100% cpu), but cpu accounting is
> > wrong (cpu is 100% idle, in the per-core view as well).
> > 
> > If I understand correctly, the stats are updated by account_user_time()  
> > -> task_group_account_field() but there is no call for it in case of  
> > offloading (it is called from irqtime_account_process_tick,
> > account_process_tick, vtime_user_exit).  
> 
> Ah I forgot about kcpustat accounting. I remember I wanted to fix that a
> few years ago but I forgot about it when I removed the last tick. That
> thing was lurking behind 1Hz.
> 
> > 
> > Moreover, task_group_account_field() uses __this_cpu_add() which will be
> > wrong for offloading.
> > 
> > For testing I used kcpustat_cpu(task_cpu(p)) in
> > task_group_account_field() and added call account_user_time(curr, delta)
> > to the sched_tick_remote() what fixes it for me, but what would be the
> > proper fix?  
> 
> Yeah unfortunately that's unsafe. Task accounting is not designed for remote
> update. You could race with an update from another CPU, especially the local
> updater.
> 
> I fear we need to take the same approach than task cputime, which is using a seqcount
> for updates. Then the reader would fetch the kcpustat values + the delta
> vtime from the task executing.
> 
> Things can get complicated once we dive into corner cases: CPUTIME_IRQ,
> CPUTIME_SOFTIRQ, and CPUTIME_STEAL. At least we don't need to care about CPUTIME_IDLE
> and CPUTIME_IOWAIT that have their own delta.
> 
> I'm trying that.

Cool! Needless to say, but we can help testing once you have patches.