From mboxrd@z Thu Jan  1 00:00:00 1970
From: oleg@redhat.com (Oleg Nesterov)
Date: Thu, 31 Jul 2014 21:05:18 +0200
Subject: TIF_NOHZ can escape nonhz mask? (Was: [PATCH v3 6/8] x86:
 Split syscall_trace_enter into two phases)
In-Reply-To: <CAFTL4hyHh3Bw0eeJe9q50HVrt=B-zgmyu6C_hy+RoW21kQEJtg@mail.gmail.com>
References: <20140728192209.GA26017@localhost.localdomain>
 <20140729175414.GA3289@redhat.com>
 <20140730163516.GC18158@localhost.localdomain>
 <20140730174630.GA30862@redhat.com>
 <20140731003034.GA32078@localhost.localdomain>
 <20140731160353.GA14772@redhat.com>
 <20140731171329.GD7842@localhost.localdomain>
 <20140731181230.GA18695@redhat.com>
 <20140731184729.GA12296@localhost.localdomain>
 <CAFTL4hyHh3Bw0eeJe9q50HVrt=B-zgmyu6C_hy+RoW21kQEJtg@mail.gmail.com>
Message-ID: <20140731190518.GA21938@redhat.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On 07/31, Frederic Weisbecker wrote:
>
> 2014-07-31 20:47 GMT+02:00 Frederic Weisbecker <fweisbec@gmail.com>:
> > 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.