Re: TIF_NOHZ can escape nonhz mask? (Was: [PATCH v3 6/8] x86: Split syscall_trace_enter into two phases)

[Oleg Nesterov]

On 07/31, Frederic Weisbecker wrote:
>
> 2014-07-31 20:47 GMT+02:00 Frederic Weisbecker <fweisbec@xxxxxxxxx>:
> > On Thu, Jul 31, 2014 at 08:12:30PM +0200, Oleg Nesterov wrote:
> >> On 07/31, Frederic Weisbecker wrote:
> > No, because preempt_schedule_irq() does the ctx_state save and restore with
> > exception_enter/exception_exit.
>
> Similar thing happens with schedule_user().
>
> preempt_schedule_irq() handles kernel preemption and schedule_user()
> the user preemption. On both cases we save and restore the context
> tracking state.
>
> This might be the missing piece you were missing :)

YYYYYEEEEESSSS, thanks!!

And in fact I was going to suggest to add this logic into preempt schedule
paths to improve the situation if we can't make TIF_NOHZ per-cpu.

But Frederic, perhaps I'll return here tomorrow with another question, it
is too late for me now ;)

Thanks!

Oleg.

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