linux-kernel - Re: TIF_NOHZ can escape nonhz mask? (Was: [PATCH v3 6/8] x86: Split syscall_trace

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Message-ID: <20140731190518.GA21938@redhat.com>
Date:	Thu, 31 Jul 2014 21:05:18 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	"<linux-arm-kernel@...ts.infradead.org>" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	LSM List <linux-security-module@...r.kernel.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: TIF_NOHZ can escape nonhz mask? (Was: [PATCH v3 6/8] x86:
	Split syscall_trace_enter into two phases)

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

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