Date:   Tue, 14 Aug 2018 14:41:46 -0400
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     NeilBrown <neilb@...e.com>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Martin Wilck <mwilck@...e.de>, linux-fsdevel@...r.kernel.org,
        Frank Filz <ffilzlnx@...dspring.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5 v2] locks: avoid thundering-herd wake-ups

This version looks correct to me, and simpler.  I'll be curious to hear
whatever you learn from testing!

--b.

On Tue, Aug 14, 2018 at 01:56:51PM +1000, NeilBrown wrote:
> 
> V2, which added wake_non_conflicts() was more broken than V1 - as
> Bruce explained there is no transitivity in the blocking relation
> between locks.
> So this series takes a simpler approach.
> It still attached waiters between other waiters as necessary to ensure
> that:
>   - a waiter is blocked by it's parent (fl->blocker) and all further
>     ancestors, and
>   - the list of waiters on fl_blocked are mutually non-conflicting.
> 
> When a lock (the root of a tree of requests) is released, only its
> immediate children (fl_blocked) are woken.
> When any lock is woken (either because its fl_blocker was released
> to due to a signal or similar) it with either:
>  - be granted
>  - be aborted
>  - be re-queued beneath some other lock.
> 
> In the first case tree of blocked locks is moved across to the newly
> created lock, and the invariants still hold.
> In the order two cases, the tree or blocked waiters are all detached
> and woken.
> 
> Note that this series has not received much testing yet.
> 
> Original description:
> If you have a many-core machine, and have many threads all wanting to
> briefly lock a give file (udev is known to do this), you can get quite
> poor performance.
> 
> When one thread releases a lock, it wakes up all other threads that
> are waiting (classic thundering-herd) - one will get the lock and the
> others go to sleep.
> When you have few cores, this is not very noticeable: by the time the
> 4th or 5th thread gets enough CPU time to try to claim the lock, the
> earlier threads have claimed it, done what was needed, and released.
> With 50+ cores, the contention can easily be measured.
> 
> This patchset creates a tree of pending lock request in which siblings
> don't conflict and each lock request does conflict with its parent.
> When a lock is released, only requests which don't conflict with each
> other a woken.
> 
> Testing shows that lock-acquisitions-per-second is now fairly stable even
> as number of contending process goes to 1000.  Without this patch,
> locks-per-second drops off steeply after a few 10s of processes.
> 
> There is a small cost to this extra complexity.
> At 20 processes running a particular test on 72 cores, the lock
> acquisitions per second drops from 1.8 million to 1.4 million with
> this patch.  For 100 processes, this patch still provides 1.4 million
> while without this patch there are about 700,000.
> 
> NeilBrown
> 
> ---
> 
> NeilBrown (5):
>       fs/locks: rename some lists and pointers.
>       fs/locks: split out __locks_wake_up_blocks().
>       fs/locks: allow a lock request to block other requests.
>       fs/locks: change all *_conflict() functions to return bool.
>       fs/locks: create a tree of dependent requests.
> 
> 
>  fs/cifs/file.c                  |    2 -
>  fs/locks.c                      |  156 ++++++++++++++++++++++++++-------------
>  include/linux/fs.h              |    7 +-
>  include/trace/events/filelock.h |   16 ++--
>  4 files changed, 119 insertions(+), 62 deletions(-)
> 
> --
> Signature

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