linux-kernel - Re: [PATCH v2 2/4] mm: vmalloc: use rwsem, mutex for vmap_area_lock and vmap

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Date:   Wed, 22 Mar 2023 19:01:59 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Dave Chinner <david@...morbit.com>,
        Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Liu Shixin <liushixin2@...wei.com>,
        Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH v2 2/4] mm: vmalloc: use rwsem, mutex for vmap_area_lock
 and vmap_block->lock

On Wed, Mar 22, 2023 at 05:47:28PM +0000, Matthew Wilcox wrote:
> 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@...38:~/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.
>
Then it has nothing to do with vmalloc contention(i mean global KVA allocator), IMHO.
Unless:

<snip>
void *vm_map_ram(struct page **pages, unsigned int count, int node)
{
	unsigned long size = (unsigned long)count << PAGE_SHIFT;
	unsigned long addr;
	void *mem;

	if (likely(count <= VMAP_MAX_ALLOC)) {
		mem = vb_alloc(size, GFP_KERNEL);
		if (IS_ERR(mem))
			return NULL;
		addr = (unsigned long)mem;
	} else {
		struct vmap_area *va;
		va = alloc_vmap_area(size, PAGE_SIZE,
				VMALLOC_START, VMALLOC_END, node, GFP_KERNEL);
		if (IS_ERR(va))
			return NULL;
<snip>

number of pages > VMAP_MAX_ALLOC.

That is why i have asked about workloads because i would like to understand
where a "problem" is. A vm_map_ram() access the global vmap space but it happens 
when a new vmap block is required and i also think it is not a problem.

But who knows, therefore it makes sense to have a lock at workload.

--
Uladzislau Rezki