linux-kernel - Re: [PATCH RFC] ext4: skip concurrent inode updates in lazytime optimization

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Message-ID: <20200129221546.GB303030@mit.edu>
Date:   Wed, 29 Jan 2020 17:15:46 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        Dmitry Monakhov <dmtrmonakhov@...dex-team.ru>
Subject: Re: [PATCH RFC] ext4: skip concurrent inode updates in lazytime
 optimization

On Wed, Jan 29, 2020 at 06:44:05PM +0300, Konstantin Khlebnikov wrote:
> Function ext4_update_other_inodes_time() implements optimization which
> opportunistically updates times for inodes within same inode table block.
> 
> For now	concurrent inode lookup by number does not scale well because
> inode hash table is protected with single spinlock. It could become very
> hot at concurrent writes to fast nvme when inode cache has enough inodes.
> 
> Probably someday inode hash will become searchable under RCU.
> (see linked patchset by David Howells)
> 
> Let's skip concurrent updates instead of wasting cpu time at spinlock.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> Link: https://lore.kernel.org/lkml/155620449631.4720.8762546550728087460.stgit@warthog.procyon.org.uk/

Hmm.... I wonder what Al thinks of adding a varaint of
find_inode_nowait() which uses tries to grab the inode_hash_lock()
using a trylock, and returns ERR_PTR(-EAGAIN) if the attempt to grab
the lock fails.

This might be better since it will prevent other conflicts between
ext4_update_other_inodes_time() and other attempts to lookup inodes
which can't be skipped if things are busy.

      	       	       	  	     - Ted