linux-kernel - Re: [PATCH 4/6] block: cfq: make the io contect sharing lockless

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Message-ID: <20080124073606.GN6258@kernel.dk>
Date:	Thu, 24 Jan 2008 08:36:06 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, knikanth@...ell.com,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH 4/6] block: cfq: make the io contect sharing lockless

On Wed, Jan 23 2008, Andrew Morton wrote:
> > On Tue, 22 Jan 2008 10:49:19 +0100 Jens Axboe <jens.axboe@...cle.com> wrote:
> > The io context sharing introduced a per-ioc spinlock, that would protect
> > the cfq io context lookup. That is a regression from the original, since
> > we never needed any locking there because the ioc/cic were process private.
> > 
> > The cic lookup is changed from an rbtree construct to a radix tree, which
> > we can then use RCU to make the reader side lockless. That is the performance
> > critical path, modifying the radix tree is only done on process creation
> > (when that process first does IO, actually) and on process exit (if that
> > process has done IO).
> 
> Perhaps Paul would review the rcu usage here sometime?

That would indeed be awesome :-)

> > +/*
> > + * Add cic into ioc, using cfqd as the search key. This enables us to lookup
> > + * the process specific cfq io context when entered from the block layer.
> > + * Also adds the cic to a per-cfqd list, used when this queue is removed.
> > + */
> > +static inline int
> 
> There's a lot of pointless inlining in there.

Will kill.

> > +++ b/block/ll_rw_blk.c
> > @@ -3831,6 +3831,16 @@ int __init blk_dev_init(void)
> >  	return 0;
> >  }
> >  
> > +static void cfq_dtor(struct io_context *ioc)
> > +{
> > +	struct cfq_io_context *cic[1];
> > +	int r;
> > +
> > +	r = radix_tree_gang_lookup(&ioc->radix_root, (void **) cic, 0, 1);
> > +	if (r > 0)
> > +		cic[0]->dtor(ioc);
> > +}
> 
> Some comments here might help others who are wondering why we can't just
> use radix_tree_lookup().

Sure, will add a comment.

> > @@ -3900,7 +3911,7 @@ struct io_context *alloc_io_context(gfp_t gfp_flags, int node)
> >  		ret->last_waited = jiffies; /* doesn't matter... */
> >  		ret->nr_batch_requests = 0; /* because this is 0 */
> >  		ret->aic = NULL;
> > -		ret->cic_root.rb_node = NULL;
> > +		INIT_RADIX_TREE(&ret->radix_root, GFP_ATOMIC | __GFP_HIGH);
> 
> Did this need to be atomic?

It's actually only ever used with a radix_tree_preload() where the
proper gfp mask is passed, the actual radix_tree_insert() is done under
lock protecting the tree.

-- 
Jens Axboe

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