linux-kernel - Re: [PATCH-v3 1/4] idr: Percpu ida

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Date:	Mon, 26 Aug 2013 13:23:54 -0700
From:	Kent Overstreet <kmo@...erainc.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	target-devel <target-devel@...r.kernel.org>,
	lf-virt <virtualization@...ts.linux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>,
	kvm-devel <kvm@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>, Asias He <asias@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH-v3 1/4] idr: Percpu ida

On Wed, Aug 21, 2013 at 06:25:58PM +0000, Christoph Lameter wrote:
> On Fri, 16 Aug 2013, Nicholas A. Bellinger wrote:
> 
> > +	spinlock_t			lock;
> 
> Remove the spinlock.

As Andrew noted, the spinlock is needed because of tag stealing. (You
don't think I'd stick a spinlock on a percpu data structure without a
real reason, would you?)

> > +	unsigned			nr_free;
> > +	unsigned			freelist[];
> > +};
> > +
> > +static inline void move_tags(unsigned *dst, unsigned *dst_nr,
> > +			     unsigned *src, unsigned *src_nr,
> > +			     unsigned nr)
> > +{
> > +	*src_nr -= nr;
> > +	memcpy(dst + *dst_nr, src + *src_nr, sizeof(unsigned) * nr);
> > +	*dst_nr += nr;
> > +}
> > +
> 
> > +static inline unsigned alloc_local_tag(struct percpu_ida *pool,
> > +				       struct percpu_ida_cpu *tags)
> 
> Pass the __percpu offset and not the tags pointer.

Why? It just changes where the this_cpu_ptr

> 
> > +{
> > +	int tag = -ENOSPC;
> > +
> > +	spin_lock(&tags->lock);
> 
> Interupts are already disabled. Drop the spinlock.
> 
> > +	if (tags->nr_free)
> > +		tag = tags->freelist[--tags->nr_free];
> 
> You can keep this or avoid address calculation through segment prefixes.
> F.e.
> 
> if (__this_cpu_read(tags->nrfree) {
> 	int n = __this_cpu_dec_return(tags->nr_free);
> 	tag =  __this_cpu_read(tags->freelist[n]);
> }

Can you explain what the point of that change would be? It sounds like
it's preferable to do it that way and avoid this_cpu_ptr() for some
reason, but you're not explaining why.
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