linux-kernel - Re: [RFC 3/9] ext4: Add couple of more fast

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Message-ID: <20220223115313.3s73bu7p454bodvl@quack3.lan>
Date:   Wed, 23 Feb 2022 12:53:13 +0100
From:   Jan Kara <jack@...e.cz>
To:     Ritesh Harjani <riteshh@...ux.ibm.com>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
        Harshad Shirwadkar <harshadshirwadkar@...il.com>,
        Theodore Ts'o <tytso@....edu>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 3/9] ext4: Add couple of more fast_commit tracepoints

On Wed 23-02-22 15:41:59, Ritesh Harjani wrote:
> On 22/02/23 10:40AM, Jan Kara wrote:
> > On Wed 23-02-22 02:04:11, Ritesh Harjani wrote:
> > > This adds two more tracepoints for ext4_fc_track_template() &
> > > ext4_fc_cleanup() which are helpful in debugging some fast_commit issues.
> > >
> > > Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>
> >
> > So why is this more useful than trace_ext4_fc_track_range() and other
> > tracepoints? I don't think it provides any more information? What am I
> > missing?
> 
> Thanks Jan for all the reviews.
> 
> So ext4_fc_track_template() adds almost all required information
> (including the caller info) in this one trace point along with transaction tid
> which is useful for tracking issue similar to what is mentioned in Patch-9.
> 
> (race with if inode is part of two transactions tid where jbd2 full commit
> may begin for txn n-1 while inode is still in sbi->s_fc_q[MAIN])
 
I understand commit tid is interesting but cannot we just add it to
tracepoints like trace_ext4_fc_track_range() directly? It would seem useful
to have it there and when it is there, the need for
ext4_fc_track_template() is not really big. I don't care too much but
this tracepoint looked a bit superfluous to me.

> And similarly ext4_fc_cleanup() helps with that information about which tid
> completed and whether it was called from jbd2 full commit or from fast_commit.

Yeah, that one is clear.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR