linux-kernel - Re: [PATCH] percpu_ida: Use _irqsave() instead of local_irq_save() + spin

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Date:   Sat, 5 May 2018 08:10:20 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        Nicholas Bellinger <nab@...ux-iscsi.org>,
        Shaohua Li <shli@...com>,
        Kent Overstreet <kent.overstreet@...il.com>
Subject: Re: [PATCH] percpu_ida: Use _irqsave() instead of local_irq_save() +
 spin_lock

On 5/4/18 9:51 PM, Matthew Wilcox wrote:
> On Fri, May 04, 2018 at 04:22:16PM -0700, Andrew Morton wrote:
>> I'm feeling a bit hostile toward lib/percpu_ida.c in general ;) It has
>> very few users and seems rather complicated (what's with that
>> schedule() in percpu_ida_alloc?).  I'm suspecting and hoping that if
>> someone can figure out what the requirements were, this could all be
>> zapped and reimplemented using something else which we already have.
> 
> Note that I have no code in percpu_ida ... it's quite different from
> the regular IDA.  But I have noticed the stunning similarity between the
> percpu_ida and the code in lib/sbitmap.c.  I have no idea which one is
> better, but they're essentially doing the same thing.

Not sure where you see that "stunning similarity"? The sbitmap code is
basically the blk-mq tagging sparse bitmaps, abstracted into a generally
usable form. The percpu_ida design works fine for lower utilization, but
it fell apart for the tagging use case where we can easily run at full
utilization. percpu_ida has percpu caches, sbitmap gets away with just
percpu hints. These caches are why it doesn't work well for > 50%
utilization. sbitmap also supports shallow operations, and online
resizing. Outside of the sharing the same basic functionality of "give
me some free ID", I really don't see a lot of similarities. In terms of
functionality, yes, I don't think it would be hard to get rid of
percpu_ida and just replace it with sbitmap. Probably a worthwhile
pursuit.

-- 
Jens Axboe