linux-kernel - Re: [tip:x86/mm] x86/mm: Avoid superflous leave

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Message-ID: <20111206053100.GA30784@elte.hu>
Date:	Tue, 6 Dec 2011 06:31:00 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/mm] x86/mm: Avoid superflous leave_mm() in the TLB
 flush path


* Shaohua Li <shaohua.li@...el.com> wrote:

> On Tue, 2011-12-06 at 07:26 +0800, Linus Torvalds wrote:
> > On Mon, Dec 5, 2011 at 9:56 AM, tip-bot for Shaohua Li
> > <shaohua.li@...el.com> wrote:
> > >
> > > x86/mm: Avoid superflous leave_mm() in the TLB flush path
> > >
> > > If just one page VA tlb is required to be flushed and current
> > > task is in lazy TLB state, doing leave_mm() is superfluous
> > > because it flushes the whole TLB.  This can reduce some TLB
> > > miss.
> > 
> > Are you sure this is a good idea?
> > 
> > We may be *much* better off leaving the VM and avoiding future IPI's
> > than trying to flush pages one by one over and over again.
> > 
> > IPI's are expensive.
> > 
> > I don't think you thought this through, and if you *did* think it
> > through and actually have numbers that it's cheaper to stay with the
> > MM, I think you need to show those numbers.
>
> good point, I didn't think of it. It's unlikely we just flush 
> one tlb page. please drop the patch.

Dropped, thanks guys.

	Ingo
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