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

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Message-ID: <8cd31bcd-dad4-44e3-920f-299a656aea98@lucifer.local>
Date:   Tue, 21 Mar 2023 07:45:56 +0000
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Dave Chinner <david@...morbit.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>,
        Matthew Wilcox <willy@...radead.org>,
        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 Tue, Mar 21, 2023 at 06:23:39AM +0100, Uladzislau Rezki wrote:
> On Tue, Mar 21, 2023 at 12:09:12PM +1100, Dave Chinner wrote:
> > On Sun, Mar 19, 2023 at 07:09:31AM +0000, Lorenzo Stoakes wrote:
> > > vmalloc() is, by design, not permitted to be used in atomic context and
> > > already contains components which may sleep, so avoiding spin locks is not
> > > a problem from the perspective of atomic context.
> > >
> > > The global vmap_area_lock is held when the red/black tree rooted in
> > > vmap_are_root is accessed and thus is rather long-held and under
> > > potentially high contention. It is likely to be under contention for reads
> > > rather than write, so replace it with a rwsem.
> > >
> > > Each individual vmap_block->lock is likely to be held for less time but
> > > under low contention, so a mutex is not an outrageous choice here.
> > >
> > > A subset of test_vmalloc.sh performance results:-
> > >
> > > fix_size_alloc_test             0.40%
> > > full_fit_alloc_test		2.08%
> > > long_busy_list_alloc_test	0.34%
> > > random_size_alloc_test		-0.25%
> > > random_size_align_alloc_test	0.06%
> > > ...
> > > all tests cycles                0.2%
> > >
> > > This represents a tiny reduction in performance that sits barely above
> > > noise.
> >
> > 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....
> >

Hugely appreciated Dave, however I must disappoint on the rwsem as I have now
reworked my patch set to use the original locks in order to satisfy Willy's
desire to make vmalloc atomic in future, and Uladzislau's desire to not have a
~6% performance hit -
https://lore.kernel.org/all/cover.1679354384.git.lstoakes@gmail.com/

> I am working on it. I submitted a proposal how to eliminate it:
>
>
> <snip>
> Hello, LSF.
>
> Title: Introduce a per-cpu-vmap-cache to eliminate a vmap lock contention
>
> Description:
>  Currently the vmap code is not scaled to number of CPU cores in a system
>  because a global vmap space is protected by a single spinlock. Such approach
>  has a clear bottleneck if many CPUs simultaneously access to one resource.
>
>  In this talk i would like to describe a drawback, show some data related
>  to contentions and places where those occur in a code. Apart of that i
>  would like to share ideas how to eliminate it providing a few approaches
>  and compare them.
>
> Requirements:
>  * It should be a per-cpu approach;
>  * Search of freed ptrs should not interfere with other freeing(as much as we can);
>  *   - offload allocated areas(buzy ones) per-cpu;
>  * Cache ready sized objects or merge them into one big per-cpu-space(split on demand);
>  * Lazily-freed areas either drained per-cpu individually or by one CPU for all;
>  * Prefetch a fixed size in front and allocate per-cpu
>
> Goals:
>  * Implement a per-cpu way of allocation to eliminate a contention.
>
> Thanks!
> <snip>
>
> --
> Uladzislau Rezki
>

That's really awesome! I will come to that talk at LSF/MM :) being able to
sustain the lock in atomic context seems to be an aspect that is important going
forward also.