Re: [PATCH v2 2/4] mm: vmalloc: use rwsem, mutex for vmap_area_lock and vmap_block->lock

Matthew Wilcox
2023-03-22T17:47:41Z

On Wed, Mar 22, 2023 at 02:18:19PM +0100, Uladzislau Rezki wrote:
> Hello, Dave.
>
> >
> > I'm travelling right now, but give me a few days and I'll test this
> > against the XFS workloads that hammer the global vmalloc spin lock
> > really, really badly. XFS can use vm_map_ram and vmalloc really
> > heavily for metadata buffers and hit the global spin lock from every
> > CPU in the system at the same time (i.e. highly concurrent
> > workloads). vmalloc is also heavily used in the hottest path
> > throught the journal where we process and calculate delta changes to
> > several million items every second, again spread across every CPU in
> > the system at the same time.
> >
> > We really need the global spinlock to go away completely, but in the
> > mean time a shared read lock should help a little bit....
> >
> Could you please share some steps how to run your workloads in order to
> touch vmalloc() code. I would like to have a look at it in more detail
> just for understanding the workloads.
>
> Meanwhile my grep agains xfs shows:
>
> <snip>
> urezki@pc638:~/data/raid0/coding/linux-rcu.git/fs/xfs$ grep -rn vmalloc ./

You're missing:

fs/xfs/xfs_buf.c: bp->b_addr = vm_map_ram(bp->b_pages, bp->b_page_count,

which i suspect is the majority of Dave's workload. That will almost
certainly take the vb_alloc() path.