linux-kernel - Re: [PATCH 0/3] nohz: Convert tick dependency mask to atomic

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Message-ID: <20160329131441.GA16300@gmail.com>
Date:	Tue, 29 Mar 2016 15:14:41 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/3] nohz: Convert tick dependency mask to atomic_t

* Ingo Molnar <mingo@...nel.org> wrote:

> That's a totally useless analysis of task_struct::flags, while we want to 
> convert thread_info::flags...

So going over to arguing about thread_info::flags:

1)

We already have ti::flags accessors for most of the generic code. There's a few 
outliers (in the scheduler code...) which can be fixed.

2)

We could introduce a ARCH_HAS_ATOMIC_TIF flag to do per arch conversion: this 
would trigger #ifdefs in the accessors. When an architecture switches ti::flags to 
atomic_t, it also sets ARCH_HAS_ATOMIC_TIF.

I'd still do a flags => __flags rename of the field, to make the conversion easy 
and safe. So any arch that has ARCH_HAS_ATOMIC_TIF set provides an atomic_t 
thread_info::__flags field.

3)

The new accessors under ARCH_HAS_ATOMIC_TIF would use atomic.h functions to 
shuffle the thread-info flags.

4)

After one kernel release we could add:

	WARN_ONCE("please convert thread_info::flags to atomic_t!", 1);

to the old accessors to accelerate conversion.

5)

Eventually, once every architecture is converted, we could eliminate the old 
sched.c fetch_or() macro.

Thanks,

	Ingo