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

Thanks.

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