Re: [PATCH v3 01/10] sched: Provide sparsemask, a reduced contention bitmap

[Steven Sistare]

On 11/27/2018 8:19 PM, Omar Sandoval wrote:
> On Tue, Nov 27, 2018 at 10:16:56AM -0500, Steven Sistare wrote:
>> On 11/9/2018 7:50 AM, Steve Sistare wrote:
>>> From: Steve Sistare <steve.sistare@xxxxxxxxxx>
>>>
>>> Provide struct sparsemask and functions to manipulate it.  A sparsemask is
>>> a sparse bitmap.  It reduces cache contention vs the usual bitmap when many
>>> threads concurrently set, clear, and visit elements, by reducing the number
>>> of significant bits per cacheline.  For each 64 byte chunk of the mask,
>>> only the first K bits of the first word are used, and the remaining bits
>>> are ignored, where K is a creation time parameter.  Thus a sparsemask that
>>> can represent a set of N elements is approximately (N/K * 64) bytes in
>>> size.
>>>
>>> Signed-off-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
>>> ---
>>>  include/linux/sparsemask.h | 260 +++++++++++++++++++++++++++++++++++++++++++++
>>>  lib/Makefile               |   2 +-
>>>  lib/sparsemask.c           | 142 +++++++++++++++++++++++++
>>>  3 files changed, 403 insertions(+), 1 deletion(-)
>>>  create mode 100644 include/linux/sparsemask.h
>>>  create mode 100644 lib/sparsemask.c
>>
>> Hi Peter and Ingo,
>>   I need your opinion: would you prefer that I keep the new sparsemask type, 
>> or fold it into the existing sbitmap type?  There is some overlap between the 
>> two, but mostly in trivial one line functions. The main differences are:
> 
> Adding Jens and myself.
> 
>>   * sparsemask defines iterators that allow an inline loop body, like cpumask,
>>   whereas the sbitmap iterator forces us to define a callback function for
>>   the body, which is awkward.
>>
>>   * sparsemask is slightly more efficient.  The struct and variable length
>>   bitmap are allocated contiguously,
> 
> That just means you have the pointer indirection elsewhere :) The users
> of sbitmap embed it in whatever structure they have.
 
Yes, the sparsemask can be embedded in one place, but in my use case I also cache
pointers to the mask from elsewhere, and those sites incur the cost of 2 indirections
to perform bitmap operations.

>>   and sbitmap uses an extra field "depth"
>>   per bitmap cacheline.
> 
> The depth field is memory which would otherwise be unused, and it's only
> used for sbitmap_get(), so it doesn't have any cost if you're using it
> like a cpumask.
> 
>>   * The order of arguments is different for the sparsemask accessors and
>>   sbitmap accessors.  sparsemask mimics cpumask which is used extensively
>>   in the sched code.
>>
>>   * Much of the sbitmap code supports queueing, sleeping, and waking on bit
>>   allocation, which is N/A for scheduler load load balancing.  However, we
>>   can call the basic functions which do not use queueing.
>>
>> I could add the sparsemask iterators to sbitmap (90 lines), and define
>> a thin layer to change the argument order to mimic cpumask, but that
>> essentially recreates sparsemask.
> 
> We only use sbitmap_for_each_set() in a few places. Maybe a for_each()
> style macro would be cleaner for those users, too, in which case I
> wouldn't be opposed to changing it. The cpumask argument order thing is
> a annoying, though.
> 
>> Also, pushing sparsemask into sbitmap would limit our freedom to evolve the
>> type to meet the future needs of sched, as sbitmap has its own maintainer,
>> and is used by drivers, so changes to its API and ABI will be frowned upon.
> 
> It's a generic data structure, so of course Jens and I have no problem
> with changing it to meet more needs :) Personally, I'd prefer to only
> have one datastructure for this, but I suppose it depends on whether
> Peter and Ingo think the argument order is important enough.

The argument order is a minor thing, not a blocker to adoption, but efficiency 
is important in the core scheduler code.  I actually did the work to write a
for_each macro with inline body to sbitmap, and converted my patches to use sbitmap.
But then I noticed your very recent patch adding the cleared word to each cacheline, 
which must be loaded and ANDed with each bitset word in the for_each traversal,
adding more overhead which we don't need for the scheduler use case, on top of the
extra indirection noted above. You might add more such things in the future (a
"deferred set" word?) to support the needs of the block drivers who are the 
intended clients of sbitmap.

Your sbitmap is more than a simple bitmap abstraction, and for the scheduler we
just need simple.  Therefore, I propose to trim sparsemask to the bare minimum,
and move it to kernel/sched for use 
by sched only.
  It was 400 lines, but will
be 200, and 80 of those are comments.

If anyone objects, please speak now.

- Steve

>> FWIW, here is the amount of code involved:
>>
>> include/linux/sbitmap.h
>>   250 lines basic operations
>>   284 lines for queueing
>>   ---
>>   534 lines total
>>
>> lib/sbitmap.c
>>   201 lines basic operations
>>   380 lines for queueing
>>   ---
>>   581 lines total
>>
>> include/linux/sparsemask.h
>>   260 lines total
>>   https://lkml.org/lkml/2018/11/9/1176
>>
>> lib/sparsemask.c
>>   142 lines total
>>   https://lkml.org/lkml/2018/11/9/1176
>>
>> - Steve