Re: [PATCH v2 02/16] ext4: remove unnecessary s_mb_last_start

Jan Kara
2025-06-30T07:31:53Z

On Mon 30-06-25 11:32:16, Baokun Li wrote:
> On 2025/6/28 2:15, Jan Kara wrote:
> > On Mon 23-06-25 15:32:50, Baokun Li wrote:
> > > ac->ac_g_ex.fe_start is only used in ext4_mb_find_by_goal(), but STREAM
> > > ALLOC is activated after ext4_mb_find_by_goal() fails, so there's no need
> > > to update ac->ac_g_ex.fe_start, remove the unnecessary s_mb_last_start.
> > >
> > > Signed-off-by: Baokun Li <libaokun1@huawei.com>
> > I'd just note that ac->ac_g_ex.fe_start is also used in
> > ext4_mb_collect_stats() so this change may impact the statistics gathered
> > there. OTOH it is questionable whether we even want to account streaming
> > allocation as a goal hit... Anyway, I'm fine with this, I'd just mention it
> > in the changelog.
> Yes, I missed ext4_mb_collect_stats(). However, instead of explaining
> it in the changelog, I think it would be better to move the current
> s_bal_goals update to inside or after ext4_mb_find_by_goal().
>
> Then, we could add another variable, such as s_bal_stream_goals, to
> represent the hit count for global goals. This kind of statistic would
> help us fine-tune the logic for optimizing inode goals and global goals.
>
> What are your thoughts on this?

Sure that sounds good to me.

> > > @@ -2849,7 +2848,6 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> > > /* TBD: may be hot point */
> > > spin_lock(&sbi->s_md_lock);
> > > ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
> > > - ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
> > Maybe reset ac->ac_g_ex.fe_start to 0 instead of leaving it at some random
> > value? Just for the sake of defensive programming...
> >
> ac->ac_g_ex.fe_start holds the inode goal's start position, not a random
> value. It's unused after ext4_mb_find_by_goal() (if s_bal_stream_goals is
> added). Thus, I see no need for further modification. We can always re-add
> it if future requirements change.

Yeah, I was imprecise. It is not a random value. But it is not an offset in
the group we are now setting. Therefore I'd still prefer to reset fe_start
to 0 (or some invalid value like -1 to catch unexpected use).

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