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From: Omar Sandoval <osandov@osandov.com>
To: Steven Sistare <steven.sistare@oracle.com>
Cc: mingo@redhat.com, peterz@infradead.org,
	subhra.mazumdar@oracle.com, dhaval.giani@oracle.com,
	daniel.m.jordan@oracle.com, pavel.tatashin@microsoft.com,
	matt@codeblueprint.co.uk, umgwanakikbuti@gmail.com,
	riel@redhat.com, jbacik@fb.com, juri.lelli@redhat.com,
	valentin.schneider@arm.com, vincent.guittot@linaro.org,
	quentin.perret@arm.com, linux-kernel@vger.kernel.org,
	Jens Axboe <axboe@fb.com>
Subject: Re: [PATCH v3 01/10] sched: Provide sparsemask, a reduced contention bitmap
Date: Tue, 27 Nov 2018 17:19:04 -0800	[thread overview]
Message-ID: <20181128011904.GR846@vader> (raw)
In-Reply-To: <7a3e87ac-db63-27c5-8490-2330637e59b1@oracle.com>

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@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.sistare@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.

>   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.

> 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

  reply	other threads:[~2018-11-28  1:19 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-09 12:50 [PATCH v3 00/10] steal tasks to improve CPU utilization Steve Sistare
2018-11-09 12:50 ` [PATCH v3 01/10] sched: Provide sparsemask, a reduced contention bitmap Steve Sistare
2018-11-27 15:16   ` Steven Sistare
2018-11-28  1:19     ` Omar Sandoval [this message]
2018-12-06 16:07       ` Steven Sistare
2018-12-06 18:19         ` Omar Sandoval
2018-11-09 12:50 ` [PATCH v3 02/10] sched/topology: Provide hooks to allocate data shared per LLC Steve Sistare
2018-11-09 12:50 ` [PATCH v3 03/10] sched/topology: Provide cfs_overload_cpus bitmap Steve Sistare
2018-11-09 17:38   ` Valentin Schneider
2018-11-19 17:32     ` Steven Sistare
2018-11-20 12:52       ` Valentin Schneider
2018-11-12 16:42   ` Valentin Schneider
2018-11-19 17:33     ` Steven Sistare
2018-11-20 12:42       ` Valentin Schneider
2018-11-26 19:06         ` Steven Sistare
2018-12-03 16:56           ` Valentin Schneider
2018-12-06 16:40             ` Steven Sistare
2018-12-06 17:28               ` Valentin Schneider
2018-11-09 12:50 ` [PATCH v3 04/10] sched/fair: Dynamically update cfs_overload_cpus Steve Sistare
2018-11-09 12:50 ` [PATCH v3 05/10] sched/fair: Hoist idle_stamp up from idle_balance Steve Sistare
2018-11-09 19:07   ` Valentin Schneider
2018-11-19 17:31     ` Steven Sistare
2018-11-20 10:24       ` Valentin Schneider
2018-11-09 12:50 ` [PATCH v3 06/10] sched/fair: Generalize the detach_task interface Steve Sistare
2018-11-09 12:50 ` [PATCH v3 07/10] sched/fair: Provide can_migrate_task_llc Steve Sistare
2018-11-09 12:50 ` [PATCH v3 08/10] sched/fair: Steal work from an overloaded CPU when CPU goes idle Steve Sistare
2018-11-09 12:50 ` [PATCH v3 09/10] sched/fair: disable stealing if too many NUMA nodes Steve Sistare
2018-11-09 12:50 ` [PATCH v3 10/10] sched/fair: Provide idle search schedstats Steve Sistare
2018-11-10 17:08   ` kbuild test robot
2018-11-09 15:02 ` hackbench run scripts Steven Sistare

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