linux-kernel - Re: [PATCH 05/10] nohz: New tick dependency mask

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Message-ID: <55B2794A.8040707@ezchip.com>
Date:	Fri, 24 Jul 2015 13:43:38 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 05/10] nohz: New tick dependency mask

On 07/24/2015 01:16 PM, Frederic Weisbecker wrote:
> On Fri, Jul 24, 2015 at 12:55:35PM -0400, Chris Metcalf wrote:
>> On 07/23/2015 12:42 PM, Frederic Weisbecker wrote:
>>> +unsigned long __tick_nohz_set_tick_dependency(enum tick_dependency_bit bit,
>>> +					      unsigned long *dep)
>>> +{
>>> +	unsigned long prev;
>>> +	unsigned long old = *dep;
>>> +	unsigned long mask = BIT_MASK(bit);
>>> +
>>> +	while ((prev = cmpxchg(dep, old, old | mask)) != old) {
>>> +		old = prev;
>>> +		cpu_relax();
>>> +	}
>>> +
>>> +	return prev;
>>> +}
>> Why not use set_bit() here?  It is suitably atomic.
> Because I don't want to send an IPI if the CPU already had bits set in
> the dependency.
>
> Ideally I need something like test_and_set_bit() but which returns the
> whole previous value and not just the previous value of the bit.

Ah, of course.  Peter, maybe we need atomic_or_return() as part
of your new atomic_or/_and/_xor series?  Certainly on tilegx, and
likely other architectures, we can do better than Frederic's
cmpxchg() loop.

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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