linux-kernel - Re: [PATCH] sched: Remove synchronize rcu/sched calls from _cpu

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Message-ID: <20140725124459.GM20603@laptop.programming.kicks-ass.net>
Date:	Fri, 25 Jul 2014 14:44:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kaushal Kumar <kaushalk@...eaurora.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>,
	Arun Bharadwaj <abharadw@...eaurora.org>,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] sched: Remove synchronize rcu/sched calls from _cpu_down

On Fri, Jul 25, 2014 at 05:26:08PM +0530, Kaushal Kumar wrote:
> synchronize_rcu calls fail hot-unplug latency tests since
> wait_rcu_gp can have very high latency, at times up to few
> hundred of millisecs.

So bloody what?

> These calls were earlier removed for the same reason by the
> commit 9ee349ad6d326df3633d43f54202427295999c47, but got added
> back as part of commit c4575f83b9fe87bb57b737bc8a667f746a20320a.

fatal: bad object c4575f83b9fe87bb57b737bc8a667f746a20320

> Moreover there should be no need for sync_sched() in _cpu_down
> as stop_machine() provides that barrier implicitly.

We should not rely on that, but also you've removed synchronize_rcu()
and this is not at all provided by stop_machine().

So NAK.
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