linux-kernel - Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish

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Message-ID: <20150929075541.GA30097@gmail.com>
Date:	Tue, 29 Sep 2015 09:55:41 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, Jens Axboe <axboe@...com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>, Dave Chinner <david@...morbit.com>,
	Jan Kara <jack@...e.cz>, Josef Bacik <jbacik@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Neil Brown <neilb@...e.de>, Christoph Hellwig <hch@....de>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Mon, Sep 28, 2015 at 10:47 AM, Peter Zijlstra <peterz@...radead.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.

cond_preempt() perhaps? That would allude to preempt_schedule() and such, and 
would make it clearer that it's supposed to be an invariant on the sleep state 
(which schedule() is not).

Thanks,

	Ingo
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