linux-kernel - Re: [RFC][PATCH 2/8] sched,idle,tile: Switch from TS_POLLING to TIF_POLLING

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Date:	Fri, 11 Apr 2014 11:15:14 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	Peter Zijlstra <peterz@...radead.org>, <mingo@...nel.org>,
	<tglx@...utronix.de>, <luto@...capital.net>,
	<nicolas.pitre@...aro.org>, <daniel.lezcano@...aro.org>,
	<umgwanakikbuti@...il.com>
CC:	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 2/8] sched,idle,tile: Switch from TS_POLLING to TIF_POLLING_NRFLAG

On 4/11/2014 9:42 AM, Peter Zijlstra wrote:
> Standardize the idle polling indicator to TIF_POLLING_NRFLAG such that
> both TIF_NEED_RESCHED and TIF_POLLING_NRFLAG are in the same word.
> This will allow us, using fetch_or(), to both set NEED_RESCHED and
> check for POLLING_NRFLAG in a single operation and avoid pointless
> wakeups.
>
> Changing from the non-atomic thread_info::status flags to the atomic
> thread_info::flags shouldn't be a big issue since most polling state
> changes were followed/preceded by a full memory barrier anyway.
>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Chris Metcalf <cmetcalf@...era.com>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> ---
>  arch/tile/include/asm/thread_info.h |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

Acked-by: Chris Metcalf <cmetcalf@...era.com>

It's unfortunate that you're rolling the fetch_or() unconditionally out of cmpxchg(), since some architectures (like tilegx) do have support for "fetchor" as a primitive atomic operation.  Currently the tilegx fetchor is only exposed to architecture-independent code via set_bit() and test_and_set_bit(), but perhaps it's worth having an atomic_or() in atomic.h so architectures can choose to optimize it?

-- 
Chris Metcalf, Tilera Corp.
http://www.tilera.com

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