linux-kernel - Re: [RFC PATCH 07/18] kthread: Make iterant kthreads freezable by default

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Date:	Mon, 15 Jun 2015 11:28:38 +0200
From:	Petr Mladek <pmladek@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Richard Weinberger <richard@....at>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Woodhouse <dwmw2@...radead.org>,
	linux-mtd@...ts.infradead.org,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Anna Schumaker <anna.schumaker@...app.com>,
	linux-nfs@...r.kernel.org, Chris Mason <clm@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Borislav Petkov <bp@...e.de>,
	Michal Hocko <mhocko@...e.cz>, live-patching@...r.kernel.org,
	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 07/18] kthread: Make iterant kthreads freezable by
 default

On Sat 2015-06-13 18:22:22, Tejun Heo wrote:
> > I try to better understand why freezer is considered to be a blunt
> > tool. Is it because it is a generic API, try_to_freeze() is put on
> > "random" locations, so that it does not define the safe point
> > precisely enough?
>
> Not that.  I don't know how to explain it better.  Hmmm... okay, let's
> say there's a shared queue Q and N users o fit.  If you wanna make Q
> empty and keep it that way for a while, the right thing to do is
> blocking new queueing and then wait till Q drains - you choke the
> entity that you wanna control.
>
> Instead of that, freezer is trying to block the "N" users part.  In
> majority of cases, it blocks enough but it's pretty difficult to be
> sure whether you actually got all N of them (as some of them may not
> involve kthreads at all or unfreezable kthreads might end up doing
> those operations too on corner cases) and it's also not that clear
> whether blocking the N users actually make Q empty.  Maybe there are
> things which can be in flight asynchronously on Q even all its N users
> are blocked.  This is inherently finicky.

I feel convinced that it does not make sense to make kthreads
freezable by default and that we should not use it when not
necessary.

Thanks a lot for patience and so detailed explanation.

Best Regards,
Petr
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