linux-kernel - Re: [PATCH] sched: Transform resched_task() into resched

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Message-ID: <53AFC6FD.5070508@yandex.ru>
Date:	Sun, 29 Jun 2014 11:57:49 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Andreas Mohr <andi@...as.de>
CC:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	mingo@...hat.com
Subject: Re: [PATCH] sched: Transform resched_task() into resched_curr()

Hi, Andreas,

On 29.06.2014 11:20, Andreas Mohr wrote:
> Hi,
> 
> I cannot speak too much about scheduler specifics, but from a structural POV
> I'm unsure about such a change (into this direction).
> 
> We seem to be going from a nicely fine-grained function
> (task-struct-specific, and thus operating on task scope alone,
> except for interesting lockdep_assert_held() outer-env validation-only parts)
> to one which has a *broader* scope (namely, wholly rq-parameterized),
> thus now drawing the rq dependency into the equation:
> this patch introduces access to rq->curr specifics *within
> function implementation* (as the first measure within a function,
> which in itself might be considered a smell),
> and it needlessly widens the scope of concerns of this handler
> by now enabling full access to any rq struct members there -
> we'll then end up with the next guy introducing
> some strange dependency on other rq parts within this handler
> which that guy would not have been tempted to do in the first place
> if it had remained strictly task-based......
> 
> I'd wager that the size benefit possibly dominantly stems from
> getting rid of rq->curr indirection lookup at the many user call sites.
> Thus it might be a good idea
> to instead create a non-inlined resched_curr() wrapper
> which merely forwards to resched_task(),
> to have the currently strictly task-focussed (pun intended ;) approach
> of resched_task() properly preserved.
> 
> Generally spoken, this incident and the "interesting" status quo
> of very often doing an open-coded rq->curr lookup when calling resched_task()
> could prompt a rethinking of relationship of task vs. rq,
> since by clearing up (and focussing on) design intentions,
> one could "automatically" end up
> with more elegant and thus better function implementations.

resched_curr(rq) means "to reschedule current task of the rq". It does
not reschedule rq itself.

We already have resched_cpu(), which has cpu agrument, and it's not
a task. I think this is just a similar case and we won't have any
problems because of this.

We only can reschedule the current task, and the patch underlines that fact.

> 
> 
> Thank you for your activities in the scheduler area!
> 
> Andreas Mohr
> 

Thanks,
Kirill
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