linux-kernel - Re: [PATCH v2 1/5] blk-mq: abstract tag allocation out into scale

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Date:   Thu, 8 Sep 2016 10:11:58 -0600
From:   Jens Axboe <axboe@...com>
To:     Alexei Starovoitov <ast@...com>,
        Omar Sandoval <osandov@...ndov.com>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH v2 1/5] blk-mq: abstract tag allocation out into
 scale_bitmap library

On 09/07/2016 07:12 PM, Alexei Starovoitov wrote:
> On 9/7/16 5:38 PM, Omar Sandoval wrote:
>> On Wed, Sep 07, 2016 at 05:01:56PM -0700, Alexei Starovoitov wrote:
>>> On 9/7/16 4:46 PM, Omar Sandoval wrote:
>>>> From: Omar Sandoval <osandov@...com>
>>>>
>>>> This is a generally useful data structure, so make it available to
>>>> anyone else who might want to use it. It's also a nice cleanup
>>>> separating the allocation logic from the rest of the tag handling
>>>> logic.
>>>>
>>>> The code is behind a new Kconfig option, CONFIG_SCALE_BITMAP, which is
>>>> only selected by CONFIG_BLOCK for now.
>>>>
>>>> This should be a complete noop functionality-wise.
>>>>
>>>> Signed-off-by: Omar Sandoval <osandov@...com>
>>>> ---
>>>>    MAINTAINERS                  |   1 +
>>>>    block/Kconfig                |   1 +
>>>>    block/blk-mq-tag.c           | 469
>>>> ++++++++++---------------------------------
>>>>    block/blk-mq-tag.h           |  37 +---
>>>>    block/blk-mq.c               | 113 +++--------
>>>>    block/blk-mq.h               |   9 -
>>>>    include/linux/blk-mq.h       |   9 +-
>>>>    include/linux/scale_bitmap.h | 340 +++++++++++++++++++++++++++++++
>>>>    lib/Kconfig                  |   3 +
>>>>    lib/Makefile                 |   2 +
>>>>    lib/scale_bitmap.c           | 305 ++++++++++++++++++++++++++++
>>> ...
>>>> diff --git a/include/linux/scale_bitmap.h
>>>> b/include/linux/scale_bitmap.h
>>>> new file mode 100644
>>>> index 0000000..63f712b
>>>> --- /dev/null
>>>> +++ b/include/linux/scale_bitmap.h
>>>> @@ -0,0 +1,340 @@
>>>> +/*
>>>> + * Fast and scalable bitmaps.
>>> ...
>>>> +/**
>>>> + * struct scale_bitmap_word - Word in a &struct scale_bitmap.
>>>> + */
>>>> +struct scale_bitmap_word {
>>>> +/**
>>>> + * struct scale_bitmap - Scalable bitmap.
>>>> + *
>>>> + * A &struct scale_bitmap is spread over multiple cachelines to
>>>> avoid ping-pong.
>>>> + * This trades off higher memory usage for better scalability.
>>>> + */
>>>> +struct scale_bitmap {
>>>
>>> scale_bitmap sounds odd, since 'scale' is also a verb.
>>> We also have lib/rhashtable.c:
>>>   * Resizable, Scalable, Concurrent Hash Table
>>> everything is 'scalable' nowadays.
>>
>> Agreed, I'm not a huge fan of the name.
>>
>>> May be resizable bitmap would be a better name?
>>> 'struct rbitmap'... lib/rbitmap.c ?
>>>
>>
>> Hm, the resizing operation isn't very well thought-out right now, it's
>> there because it's okay for the way blk-mq uses it, but it's definitely
>> not the point of the data structure. It's more of a cache-friendly
>> bitmap, or a sparse bitmap. `struct sbitmap`? `struct cbitmap`?
>
> yeah. naming is hard.
> I think the name ideally should indicate how this bitmap
> is different from array of bits that is already covered by
> primitives in bitmap.h
> Is it because the user can wait on the bit or because it's
> smp aware? sort of percpu? I think that's the main trick how
> it achieves good concurrent set/get access, right?
> struct pcpu_bitmap ?
> struct sbitmap is fine too.

It's not a true percpu bitmap. Rather it's a sparse bitmap, that
provides some nice cache behavior through the nature of the sparseness.
The percpu hinting helps with that. sbitmap might work, S for scale
and/or sparse.

No name is going to convey what is special about it, but luckily Omar
did a great job documenting it while pulling it out of blk-mq-tag. So
I'm fine with just calling it sbitmap. I'll be pronouncing it like
"spitmap".

-- 
Jens Axboe