linux-kernel - Re: [PATCH v10 1/4] cgroups: use bitmask to filter for_each

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Date:	Wed, 22 Apr 2015 12:02:58 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Aleksa Sarai <cyphar@...har.com>, lizefan@...wei.com,
	mingo@...hat.com, richard@....at, fweisbec@...il.com,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH v10 1/4] cgroups: use bitmask to filter for_each_subsys

Hey,

On Wed, Apr 22, 2015 at 05:42:12PM +0200, Peter Zijlstra wrote:
> If, as per the below you want to use the bitmap ops; it needs be
> unsigned long.

Ah, right.

> > >  static struct cftype cgroup_dfl_base_files[];
> > > +#define for_each_subsys_which(ss_mask, ss, ssid) \
> > > +	for_each_subsys((ss), (ssid)) \
> > > +		if ((ss_mask) & (1 << (ssid)))
> > 
> > Maybe using for_each_set_bit() is better?
> > 
> > #define for_each_subsys_which(ss_mask, ss, ssid)		\
> > 	for_each_set_bit(ssid, &(ss_mask), CGROUP_SUBSYS_COUNT)	\
> > 		if ((ss) = group_subsys[ssid] && false)		\
> > 			;					\
> > 		else
> 
> Clever that ;-)

Thanks.  It kinda bothers me that for_each_set_bit() doesn't collapse
to combo of ffs() + clearing bit off of a temp mask when size is const
and <= ulong, which would be quite a bit lighter.  Right now it'd be
calling into generic find_first/next_bit() functions unconditionally.
Ah well, it can be optimized later.

-- 
tejun
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