linux-kernel - Re: [PATCH v2 3/4] x86, mwaitt: introduce mwaix delay with a configurable timer

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Date:	Fri, 12 Jun 2015 16:15:23 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...e.de>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Aaron Lu <aaron.lu@...el.com>, X86 ML <x86@...nel.org>,
	Tony Li <tony.li@....com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Suravee Suthikulanit <suravee.suthikulpanit@....com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Len Brown <lenb@...nel.org>, Ken Xue <ken.xue@....com>,
	Huang Rui <ray.huang@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 3/4] x86, mwaitt: introduce mwaix delay with a
 configurable timer

On Jun 12, 2015 1:43 AM, "Borislav Petkov" <bp@...e.de> wrote:
>
> On Tue, Jun 09, 2015 at 09:46:52AM -0700, Andy Lutomirski wrote:
> > I don't like this hack.  The compiler is entirely within is rights to
> > poke addr's cacheline (i.e. the stack) between the two instructions.
> > I'd suggest either making the thing a full cacheline long or using a
> > single asm statement.
>
> How about this:
>
>         /*
>          * This should be a memory location in a cache line which is
>          * unlikely to be touched by other processors.  The actual
>          * content is immaterial as it is not actually modified in any way.
>          */
>         mwait_ptr = &current_thread_info()->flags;
>
> and then
>
>         __monitor(mwait_ptr, 0, 0);
>
> We already do this in mwait_play_dead().
>
> However, am I even correct in assuming that ->flags won't really be
> touched as we're doing delay() and nothing pokes into current anyway?

We poke flags remotely, but not frequently enough for this to be a
problem.  However, I don't know that touching current in udelay is
okay.

How about some read-mostly percpu variable, such as cpu_tss?

--Andy
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