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Sender: Tejun Heo <htejun@gmail.com>
Date: Mon, 26 Feb 2024 08:38:55 -1000
From: Tejun Heo <tj@kernel.org>
To: Allen <allen.lkml@gmail.com>
Cc: jiangshanlai@gmail.com, torvalds@linux-foundation.org,
	linux-kernel@vger.kernel.org, kernel-team@meta.com
Subject: Re: [PATCHSET wq/for-6.9,6.10] workqueue: Implement
 disable/enable_work()
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Hello,

Sorry about the late reply.

On Thu, Feb 22, 2024 at 12:26:35PM -0800, Allen wrote:
>  I do understand the work bits, but I don't fully understand the
> concept of colors
> (work_color, flush_color). A swift overview of it would be highly appreciated.

That's just to group work items for flush_workqueue(). Let's say there are
work items which are queued in sequence, 1 to 8. Let's also say that there
are three flush_workqueue() attempts which take place after work item 2, 4
and 6. Then, the flush colors A, B, C, D are assigned so that:

  work items        1     2     3     4     5     6     7     8
  flush_workqueue            ^           ^           ^
                    \-----/     \-----/     \-----/     \------~~~
  flush_color          A           B           C            D

and the flush_workqueue() code waits for its preceding colors to be drained
to implement flushing.

It's just a way to keep track of the number of work items in flight which
were issued before a certain point in time. It's expensive to do that for
arbitrary number of points in time, so it just has N slots for groups and
calls them flush colors.

This shouldn't really matter for the bh conversion given that everyone
operates on individual work item.

Thanks.

-- 
tejun