Messages in this thread |  | | Subject | Re: [PATCH] percpu_ida: Use _irqsave() instead of local_irq_save() + spin_lock | From | Jens Axboe <> | Date | Sat, 5 May 2018 08:10:20 -0600 |
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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
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