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

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Message-ID: <20150625190800.GW19282@twins.programming.kicks-ass.net>
Date:	Thu, 25 Jun 2015 21:08:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Nicholas Mc Guire <der.herr@...r.at>
Cc:	oleg@...hat.com, paulmck@...ux.vnet.ibm.com, tj@...nel.org,
	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

On Tue, Jun 23, 2015 at 09:28:11AM +0200, Nicholas Mc Guire wrote:
> 
> A bit off-topic probably
> but maybe this should not be in kernel/locking/percpu-rwsem.c but in a
> generic percpu location as this construct is present in the core a few times
> atleast in:
>  kernel/irq/irqdesc.c:kstat_irqs

>  kernel/fork.c:nr_processes

That has an odd unsigned long vs int fail, but yes.

>  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.

> > +#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 ?

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..
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