Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()

From	Linus Torvalds <torvalds@linux-foundation.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()
Date	2015-09-28 18:10 +0200
Message-ID	<qdJa1-4l8-7@gated-at.bofh.it> (permalink)
References	(6 earlier) <qa4PM-5bi-33@gated-at.bofh.it> <qa5Vx-6Jf-59@gated-at.bofh.it> <qa6oy-7wu-29@gated-at.bofh.it> <qa6ye-7HP-33@gated-at.bofh.it> <qdHUB-2nN-7@gated-at.bofh.it>
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On Mon, Sep 28, 2015 at 10:47 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
>> It gets set by preemption - and,
>> somewhat illogically, by cond_resched().
>
> I suspect that was done to make cond_resched() (voluntary preemption)
> more robust and only have a single preemption path/logic. But all that
> was done well before I got involved.

So I think it's actually the name that is bad, not necessarily the behavior.

We tend to put "cond_resched()" (and particularly
"cond_resched_lock()") in some fairly awkward places, and it's not
always entirely clear that task->state == TASK_RUNNING there.

So the preemptive behavior of not *really* putting the task to sleep
may actually be the right one. But it is rather non-intuitive given
the name - because "cond_resched()" basically is not at all equivalent
to "if (need_resched()) schedule()", which you'd kind of expect.

An explicit schedule will actually act on the task->state, and make us
go to sleep. "cond_resched()"  really is just a "voluntary preemption
point". And I think it would be better if it got named that way.

            Linus
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Thread

Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Peter Zijlstra <peterz@infradead.org> - 2015-09-28 16:50 +0200
  Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Linus Torvalds <torvalds@linux-foundation.org> - 2015-09-28 18:10 +0200
    Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug() Ingo Molnar <mingo@kernel.org> - 2015-09-29 10:00 +0200

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