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

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Message-ID: <20150803132407.GP25159@twins.programming.kicks-ass.net>
Date:	Mon, 3 Aug 2015 15:24:07 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.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 Mon, Aug 03, 2015 at 03:05:37PM +0200, Frederic Weisbecker wrote:
> On Mon, Aug 03, 2015 at 02:43:50PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 23, 2015 at 06:42:10PM +0200, 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;
> > > +}
> > 
> > That's typically called a fetch_or(). The function name, which is
> > entirely too long, also doesn't suggest a return value.
> 
> Nice, I can export fetch_or() from sched code to atomic_fetch_or() like
> you just suggested and bring it to asm_generic if you like.

asm_generic means you _have_ to go implement it for all archs that do
not use asm_generic.

The version I have is also unsuited for the atomic name, as it deals
with all integer types that cmpxchg() can consume (and I need it that
way, because thread_info::flags does not have a consistent type across
archs (unsigned long vs unsigned int is the most common variation).

You can go add it to linux/atomic.h I suppose, with the regular #ifdef
guards we have there. Then add:

  atomic{,long,64}_fetch_{add,sub,inc,dec,or,and,xor}()

I suppose, using cmpxchg() loops and then let archs implement as we go.

> So perhaps fetch_and_or() ? Of course having "and" then "or" in the same function name
> might be confusing but after all we have functions names starting with "test" then "and".

No fetch_$op is a well known name for these things, don't go change it.
That just means people will _not_ find it.

> Concerning the nohz function name, tick_nohz is a long prefix that is very likely
> to produce too long names. Perhaps I should trim tick_nohz to just nohz.

No, then we'll get confused with the regular nohz stuff. But yes a 10
letter prefix sucks, but at least you can try and not then add another
30.
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