Date: Tue, 11 Jun 2024 12:49:53 -0400
From: Josef Bacik <josef@...icpanda.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, hch@...radead.org
Subject: Re: [PATCH v3 0/2] rcu-based inode lookup for iget*

On Tue, Jun 11, 2024 at 12:16:30PM +0200, Mateusz Guzik wrote:
> I think the appropriate blurb which needs to land here also needs to be
> in the commit message for the first patch, so here it is copy pasted
> with some modifications at the end:
> 
> [quote]
> Instantiating a new inode normally takes the global inode hash lock
> twice:
> 1. once to check if it happens to already be present
> 2. once to add it to the hash
> 
> The back-to-back lock/unlock pattern is known to degrade performance
> significantly, which is further exacerbated if the hash is heavily
> populated (long chains to walk, extending hold time). Arguably hash
> sizing and hashing algo need to be revisited, but that's beyond the
> scope of this patch.
> 
> A long term fix would introduce finer-grained locking. An attempt was
> made several times, most recently in [1], but the effort appears
> stalled.
> 
> A simpler idea which solves majority of the problem and which may be
> good enough for the time being is to use RCU for the initial lookup.
> Basic RCU support is already present in the hash. This being a temporary
> measure I tried to keep the change as small as possible.
> 
> iget_locked consumers (notably ext4) get away without any changes
> because inode comparison method is built-in.
> 
> iget5_locked and ilookup5_nowait consumers pass a custom callback. Since
> removal of locking adds more problems (inode can be changing) it's not
> safe to assume all filesystems happen to cope.  Thus iget5_locked_rcu,
> ilookup5_rcu and ilookup5_nowait_rcu get added, requiring manual
> conversion.
> 
> In order to reduce code duplication find_inode and find_inode_fast grow
> an argument indicating whether inode hash lock is held, which is passed
> down should sleeping be necessary. They always rcu_read_lock, which is
> redundant but harmless. Doing it conditionally reduces readability for
> no real gain that I can see. RCU-alike restrictions were already put on
> callbacks due to the hash spinlock being held.
> 
> There is a real cache-busting workload scanning millions of files in
> parallel (it's a backup server thing), where the initial lookup is
> guaranteed to fail resulting in the 2 lock acquires.
> 
> Implemented below is a synthehic benchmark which provides the same
> behavior. [I shall note the workload is not running on Linux, instead it
> was causing trouble elsewhere. Benchmark below was used while addressing
> said problems and was found to adequately represent the real workload.]
> 
> Total real time fluctuates by 1-2s.
> 
> With 20 threads each walking a dedicated 1000 dirs * 1000 files
> directory tree to stat(2) on a 32 core + 24GB RAM vm:
> [/quote]
> 
> Specific results:
> 
> ext4 (needed mkfs.ext4 -N 24000000):
> before:	3.77s user 890.90s system 1939% cpu 46.118 total
> after:  3.24s user 397.73s system 1858% cpu 21.581 total (-53%)
> 
> btrfs (s/iget5_locked/iget5_locked_rcu in fs/btrfs/inode.c):
> before: 3.54s user 892.30s system 1966% cpu 45.549 total
> after:  3.28s user 738.66s system 1955% cpu 37.932 total (-16.7%)
> 
> btrfs bottlenecks itself on its own locks here.
> 
> Benchmark can be found here: https://people.freebsd.org/~mjg/fstree.tgz
> 
> fs rundown is as follows:
> - ext4 patched implicitly
> - xfs does not use the inode hash
> - bcachefs is out of the picture as Kent decided to implement his own
>   inode hashing based on rhashtable, for now private to his fs.
> 
> I have not looked at others.
> 
> [1] https://lore.kernel.org/all/20231206060629.2827226-1-david@fromorbit.com/
> 

Reviewed-by: Josef Bacik <josef@...icpanda.com>

Thanks,

Josef

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