Messages in this thread | | | Date | Thu, 24 Jan 2008 08:42:52 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 4/6] block: cfq: make the io contect sharing lockless |
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On Wed, Jan 23, 2008 at 02:08:19PM -0800, Andrew Morton wrote: > > On Tue, 22 Jan 2008 10:49:19 +0100 Jens Axboe <jens.axboe@oracle.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?
I will take a look -- long plane trip about 36 hours from now. ;-)
Thanx, Paul
> > +/* > > + * 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. > > > +++ 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(). > > > +static void cfq_exit(struct io_context *ioc) > > +{ > > + struct cfq_io_context *cic[1]; > > + int r; > > + > > + rcu_read_lock(); > > + r = radix_tree_gang_lookup(&ioc->radix_root, (void **) cic, 0, 1); > > + rcu_read_unlock(); > > + > > + if (r > 0) > > + cic[0]->exit(ioc); > > +} > > ditto. > > > /* Called by the exitting task */ > > void exit_io_context(void) > > { > > struct io_context *ioc; > > - struct cfq_io_context *cic; > > > > task_lock(current); > > ioc = current->io_context; > > @@ -3876,11 +3891,7 @@ void exit_io_context(void) > > if (atomic_dec_and_test(&ioc->nr_tasks)) { > > if (ioc->aic && ioc->aic->exit) > > ioc->aic->exit(ioc->aic); > > - if (ioc->cic_root.rb_node != NULL) { > > - cic = rb_entry(rb_first(&ioc->cic_root), > > - struct cfq_io_context, rb_node); > > - cic->exit(ioc); > > - } > > + cfq_exit(ioc); > > > > put_io_context(ioc); > > } > > @@ -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? > >
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