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
