linux-kernel - Re: [RFC][PATCH 05/13] percpu-rwsem: Optimize readers and reduce global impact

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Message-ID: <20150625191701.GA5013@mtj.duckdns.org>
Date:	Thu, 25 Jun 2015 15:17:01 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Nicholas Mc Guire <der.herr@...r.at>, oleg@...hat.com,
	paulmck@...ux.vnet.ibm.com, mingo@...hat.com,
	linux-kernel@...r.kernel.org, dave@...olabs.net, riel@...hat.com,
	viro@...IV.linux.org.uk, torvalds@...ux-foundation.org
Subject: Re: [RFC][PATCH 05/13] percpu-rwsem: Optimize readers and reduce
 global impact

Hello,

On Thu, Jun 25, 2015 at 09:08:00PM +0200, Peter Zijlstra wrote:
> >  mm/memcontrol.c:mem_cgroup_read_events
> >  mm/memcontrol.c:mem_cgroup_read_stat
> 
> Those seem to be hotplug challenged. I'm thinking dropping that
> nocpu_base.count[] crap and just iterating all possible CPUs would've
> been much easier.

A patch doing that is already queued for this merge window.  IIRC,
it's included as part of cgroup writeback updates.

> > > +#define per_cpu_sum(var)                                             \
> > > +({                                                                   \
> > > +     typeof(var) __sum = 0;                                          \
> > > +     int cpu;                                                        \
> > > +     for_each_possible_cpu(cpu)                                      \
> > > +             __sum += per_cpu(var, cpu);                             \
> > > +     __sum;                                                          \
> > > +})
> > > +
> > 
> > so maybe put it into include/linux/percpu.h ?

percpu-defs.h would be the better place for it.

> Yes I can do that.
> 
> We can try and use it more after that, there seems to be loads of places
> that could use this fs/namespace.c fs/inode.c etc..

Hmmm... the only worry I have about this is people using it on u64 on
32bit machines.  CPU local ops can do split updates on lower and upper
halves and the remotely-read value will be surprising.  We have the
same issues w/ regular per_cpu accesses to but the summing function /
macro is better at giving the false sense of security.  Prolly
limiting it upto ulong size is a good idea?

Thanks.

-- 
tejun
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