linux-kernel - Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

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Date:	Thu, 7 May 2015 14:49:39 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	fweisbec@...hat.com, Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>, williams@...hat.com,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
 & enable from context tracking on syscall entry


* Ingo Molnar <mingo@...nel.org> wrote:

> The TIF_RCU_QS thing is just a fancy way for synchronize_rcu() 
> (being executed on some other CPU not doing RT work) to 
> intelligently wait for the remote (RT work doing) CPU to finish 
> executing kernel code, without polling or so.

it's basically a cheap IPI being inserted on the remote CPU.

We need the TIF_RCU_QS callback not just to wait intelligently, but 
mainly to elapse a grace period, otherwise synchronize_rcu() might not 
ever make progress: think a SCHED_FIFO task doing some kernel work, 
synchronize_rcu() stumbling upon it - but the SCHED_FIFO task 
otherwise never scheduling and never getting any timer irqs either, 
and thus never entering quiescent state.

(Cc:-ed Paul too, he might be interested in this as well.)

Thanks,

	Ingo
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