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:	Fri, 1 May 2015 18:27:31 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, williams@...hat.com,
	Andrew Lutomirski <luto@...nel.org>, fweisbec@...hat.com,
	Peter Zijlstra <peterz@...radead.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
 & enable from context tracking on syscall entry


* Rik van Riel <riel@...hat.com> wrote:

> On 05/01/2015 12:03 PM, Andy Lutomirski wrote:
> 
> > The last time I asked, the impression I got was that we needed two things:
> > 
> > 1. We can't pluck things from the RCU list without knowing whether the
> > CPU is in an RCU read-side critical section, and we can't know that
> > unless we have regular grade periods or we know that the CPU is idle.
> > To make the CPU detectably idle, we need to set a bit somewhere.
> 
> More than that. We also need a way for another CPU to identify the 
> callbacks they could run for us, without confusing them with new 
> callbacks queued after we transitioned back from USER to KERNEL 
> context.

That's easy: a simple, constant-work IPI sent to nohz-full CPUs could 
just dequeue these callbacks, if it sees that we are still in 
user-mode.

It's super easy because such an IPI would essentially be equivalent to 
a system call context from RCU's POV, if it uses this test:

	if (user_mode(regs)) {
		... pluck RCU callbacks ...
	}

That way we can have lots of overhead removed from the syscall call 
path ...

Thanks,

	Ingo
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