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

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Message-ID: <20150613084840.GA3796@pd.tnic>
Date:	Sat, 13 Jun 2015 10:48:40 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Andy Lutomirski <luto@...capital.net>
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 Fri, Jun 12, 2015 at 04:15:23PM -0700, Andy Lutomirski wrote:
> > 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?

Yeah, those look much safer since they're static and are
____cacheline_aligned_in_smp, which is exactly what we want.

I guess we can do

	__monitorx(this_cpu_ptr(&cpu_tss), 0, 0);

with a nice comment ontop why we're doing it.

asm looks ok too:

	movq	$cpu_tss, %rax	#, tcp_ptr__
	add %gs:this_cpu_off(%rip), %rax	# this_cpu_off, tcp_ptr__
	xorl	%edx, %edx	# tmp248
	movq	%rdx, %rcx	# tmp248, tmp248
#APP
# 22 "./arch/x86/include/asm/mwait.h" 1
	.byte 0x0f, 0x01, 0xc8;
# 0 "" 2
#NO_APP

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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