Re: [RRC PATCH 2/2] vfs: Use per-cpu list for superblock's inode list

[Waiman Long]

On 02/17/2016 05:37 AM, Dave Chinner wrote:
> On Tue, Feb 16, 2016 at 08:31:20PM -0500, Waiman Long wrote:
> > When many threads are trying to add or delete inode to or from
> > a superblock's s_inodes list, spinlock contention on the list can
> > become a performance bottleneck.
> >
> > This patch changes the s_inodes field to become a per-cpu list with
> > per-cpu spinlocks.
> >
> > With an exit microbenchmark that creates a large number of threads,
> > attachs many inodes to them and then exits. The runtimes of that
> > microbenchmark with 1000 threads before and after the patch on a
> > 4-socket Intel E7-4820 v3 system (40 cores, 80 threads) were as
> > follows:
> >
> >    Kernel            Elapsed Time    System Time
> >    ------            ------------    -----------
> >    Vanilla 4.5-rc4      65.29s         82m14s
> >    Patched 4.5-rc4      22.81s         23m03s
> Pretty good :)
>
> My fsmark tests usually show up a fair bit of contention - moving
> 250k inodes through the cache every second over 16p does generate a
> bit of load on the list. The patch makes the inode list add/del
> operations disappear completely from the perf profiles, and there's
> a marginal decrease in runtime (~4m40s vs 4m30s). I think the global
> lock is right on the edge of breakdown under this load, though, so
> if I was testing on a larger system I think the difference would be
> much bigger.
>
> I'll run some more testing on it, see if anything breaks.
>
> A few comments on the code follow.
>
> > @@ -1866,8 +1866,8 @@ void iterate_bdevs(void (*func)(struct block_device *, void *), void *arg)
> >   {
> >   	struct inode *inode, *old_inode = NULL;
> >
> > -	spin_lock(&blockdev_superblock->s_inode_list_lock);
> > -	list_for_each_entry(inode,&blockdev_superblock->s_inodes, i_sb_list) {
> > +	for_all_percpu_list_entries_simple(inode, percpu_lock,
> > +			blockdev_superblock->s_inodes_cpu, i_sb_list) {
> This is kind what I meant about names getting way too long. How
> about something like:
>
> #define walk_sb_inodes(inode, sb, pcpu_lock)	\
> 	for_all_percpu_list_entries_simple(inode, pcpu_lock,	\
> 					   sb->s_inodes_list, i_sb_list)
>
> #define walk_sb_inodes_end(pcpu_lock) end_all_percpu_list_entries(pcpu_lock)
>
> for brevity?

Yes, I think adding some inode specific macros in fs.h will help to make 
the patch easier to read.

> > @@ -189,7 +190,7 @@ void fsnotify_unmount_inodes(struct super_block *sb)
> >   		spin_unlock(&inode->i_lock);
> >
> >   		/* In case the dropping of a reference would nuke next_i. */
> > -		while (&next_i->i_sb_list !=&sb->s_inodes) {
> > +		while (&next_i->i_sb_list.list != percpu_head) {
> >   			spin_lock(&next_i->i_lock);
> >   			if (!(next_i->i_state&  (I_FREEING | I_WILL_FREE))&&
> >   						atomic_read(&next_i->i_count)) {
> > @@ -199,16 +200,16 @@ void fsnotify_unmount_inodes(struct super_block *sb)
> >   				break;
> >   			}
> >   			spin_unlock(&next_i->i_lock);
> > -			next_i = list_next_entry(next_i, i_sb_list);
> > +			next_i = list_next_entry(next_i, i_sb_list.list);
> pcpu_list_next_entry(next_i, i_sb_list)?

Will add that.

> > @@ -1397,9 +1398,8 @@ struct super_block {
> >   	 */
> >   	int s_stack_depth;
> >
> > -	/* s_inode_list_lock protects s_inodes */
> > -	spinlock_t		s_inode_list_lock ____cacheline_aligned_in_smp;
> > -	struct list_head	s_inodes;	/* all inodes */
> > +	/* The percpu locks protect s_inodes_cpu */
> > +	PERCPU_LIST_HEAD(s_inodes_cpu);	/* all inodes */
> There is no need to encode the type of list into the name.
> i.e. drop the "_cpu" suffix - we can see it's a percpu list from the
> declaration.

Will remove that macro.

Thanks for the review.

Cheers,
Longman