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From: Allen <allen.lkml@gmail.com>
Date: Tue, 27 Feb 2024 09:56:50 -0800
Message-ID: <CAOMdWSJdaBZahLvbEqwG_x4bnoRxLAfvmCngqyUvL9dbg0DF0w@mail.gmail.com>
Subject: Re: [PATCHSET wq/for-6.9,6.10] workqueue: Implement disable/enable_work()
To: Tejun Heo <tj@kernel.org>
Cc: jiangshanlai@gmail.com, torvalds@linux-foundation.org, 
	linux-kernel@vger.kernel.org, kernel-team@meta.com
Content-Type: text/plain; charset="UTF-8"

Tejun,
>
> Sorry about the late reply.
>

No worries :)

> 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.
>

 Thank you very much. This is very helpful.

- Allen

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