linux-kernel - Re: [PATCH 2/3] context_tracking: Inherit TIF_NOHZ through forks instead of context switches

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Message-ID: <20150402191934.GA24360@redhat.com>
Date:	Thu, 2 Apr 2015 21:19:35 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 2/3] context_tracking: Inherit TIF_NOHZ through forks
	instead of context switches

On 04/02, Peter Zijlstra wrote:
>
> On Thu, Apr 02, 2015 at 07:39:24PM +0200, Frederic Weisbecker wrote:
> > TIF_NOHZ is used by context_tracking to force syscall slow-path on every
> > task in order to track userspace roundtrips. As such, it must be set on
> > all running tasks.
> >
> > It's currently explicitly inherited through context switches. There is
> > no need to do it on this fast-path though. The flag could be simply
> > set once for all on all tasks, whether they are running or not.
> >
> > Lets do this by setting the flag to init task on early boot and let it
> > propagate through fork inheritance.
> >
>
> One must ask, what's the point of the flag if everybody must always have
> it set?

To force the slow path in the syscall paths. Unfortunately it is not
easy to change this ...

Oleg.

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