On Thu, Dec 06, 2018 at 11:07:46AM -0500, Steven Sistare wrote: > 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.sist...@oracle.com> > >>> > >>> 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.sist...@oracle.com> > >>> --- > >>> 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.
Yes, after the recent changes, I think it's reasonable to have a separate implementation for sched.