linux-kernel - Re: [RFC 04/11] ext4: Convert mballoc cr (criteria) to enum

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Message-ID: <20230420145815.rs4amtveq4v3qz6p@quack3>
Date:   Thu, 20 Apr 2023 16:58:15 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
        Theodore Ts'o <tytso@....edu>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ritesh Harjani <ritesh.list@...il.com>
Subject: Re: [RFC 04/11] ext4: Convert mballoc cr (criteria) to enum

On Thu 20-04-23 12:02:44, Ojaswin Mujoo wrote:
> On Sat, Mar 25, 2023 at 08:12:36PM +0530, Ojaswin Mujoo wrote:
> > On Thu, Mar 23, 2023 at 11:55:37AM +0100, Jan Kara wrote:
> > > On Fri 17-03-23 15:56:46, Ojaswin Mujoo wrote:
> > > > On Thu, Mar 09, 2023 at 01:11:22PM +0100, Jan Kara wrote:
> > > > > Also when going for symbolic allocator scan names maybe we could actually
> > > > > make names sensible instead of CR[0-4]? Perhaps like CR_ORDER2_ALIGNED,
> > > > > CR_BEST_LENGHT_FAST, CR_BEST_LENGTH_ALL, CR_ANY_FREE. And probably we could
> > > > > deal with ordered comparisons like in:
> > > > I like this idea, it should make the code a bit more easier to
> > > > understand. However just wondering if I should do it as a part of this
> > > > series or a separate patch since we'll be touching code all around and 
> > > > I don't want to confuse people with the noise :) 
> > > 
> > > I guess a mechanical rename should not be really confusing. It just has to
> > > be a separate patch.
> > Alright, got it.
> > > 
> > > > > 
> > > > >                 if (cr < 2 &&
> > > > >                     (!sbi->s_log_groups_per_flex ||
> > > > >                      ((group & ((1 << sbi->s_log_groups_per_flex) - 1)) != 0)) &
> > > > >                     !(ext4_has_group_desc_csum(sb) &&
> > > > >                       (gdp->bg_flags & cpu_to_le16(EXT4_BG_BLOCK_UNINIT))))
> > > > >                         return 0;
> > > > > 
> > > > > to declare CR_FAST_SCAN = 2, or something like that. What do you think?
> > > > About this, wont it be better to just use something like
> > > > 
> > > > cr < CR_BEST_LENGTH_ALL 
> > > > 
> > > > instead of defining a new CR_FAST_SCAN = 2.
> > > 
> > > Yeah, that works as well.
> > > 
> > > > The only concern is that if we add a new "fast" CR (say between
> > > > CR_BEST_LENGTH_FAST and CR_BEST_LENGTH_ALL) then we'll need to make
> > > > sure we also update CR_FAST_SCAN to 3 which is easy to miss.
> > > 
> > > Well, you have that problem with any naming scheme (and even with numbers).
> > > So as long as names are all defined together, there's reasonable chance
> > > you'll remember to verify the limits still hold :)
> > haha that's true. Anyways, I'll try a few things and see what looks
> > good. Thanks for the suggestions.
> Hey Jan,
> 
> So I was playing around with this and I prepare a patch to convert CR
> numbers to symbolic names and it looks good as far as things like these
> are concerned:
> 
>   if (cr < CR_POWER2_ALIGNED)
> 		...
> 
> However there's one problem that this numeric naming scheme is used in
> several places like struct member names, function names, traces and
> comments. The issue is that replacing it everywhere is making some of
> the names very long for example:
> 
> 	atomic_read(&sbi->s_bal_cr0_bad_suggestions));
> 
> becomes:
> 
> 	atomic_read(&sbi->s_bal_cr_power2_aligned_bad_suggestions));
> 
> And this is kind of making the code look messy at a lot of places. So
> right now there are a few options we can consider:
> 
> 1. Use symbolic names everywhere at the cost of readability

Can we maybe go with 1b) being: Use symbolic names in variables / members /
...  but shortened? Like s_bal_p2aligned_bad_suggestions? Not sure how many
things are like this but from a quick looks it seems we need to come up
with a sensible shortcut only for cr0 and cr1?

								Honza

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