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

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Message-ID: <ZBs/MGH+xUAZXNTz@casper.infradead.org>
Date:   Wed, 22 Mar 2023 17:47:28 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     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 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.