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

From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: fweisbec@redhat.com, Paolo Bonzini <pbonzini@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>, X86 ML <x86@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Mike Galbraith <umgwanakikbuti@gmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Rik van Riel <riel@redhat.com>,
	williams@redhat.com, "Paul E. McKenney" <paulmck@us.ibm.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry
Date: Thu, 7 May 2015 14:49:39 +0200	[thread overview]
Message-ID: <20150507124938.GA17675@gmail.com> (raw)
In-Reply-To: <20150507124437.GB17443@gmail.com>

* Ingo Molnar <mingo@kernel.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