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

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Date:	Thu, 02 Apr 2015 15:11:20 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>, Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/3] context_tracking: Inherit TIF_NOHZ through forks
 instead of context switches

On 04/02/2015 03:09 PM, 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?

We already test this word full of flags in the syscall
entry and exit path.

Testing this same word for an additional flag is cheaper
than testing a different variable.

See the places in entry_{32,64}.S where do_notify_resume,
syscall_trace_enter, syscall_trace_leave, etc get called.
All are called as a result of testing flags in the same
word.
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