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Subject: Re: [PATCH] percpu_ida: Use _irqsave() instead of local_irq_save() +
 spin_lock
From:   Jens Axboe <axboe@kernel.dk>
To:     Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        linux-kernel@vger.kernel.org, tglx@linutronix.de,
        Nicholas Bellinger <nab@linux-iscsi.org>,
        Shaohua Li <shli@fb.com>,
        Kent Overstreet <kent.overstreet@gmail.com>
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Date:   Sat, 5 May 2018 08:42:22 -0600
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On 5/5/18 8:10 AM, Jens Axboe wrote:
> 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.

Looks like it's just the target code using it. I'll spin a patch
to convert it to sbitmap.

-- 
Jens Axboe