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

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Date:	Tue, 06 Dec 2011 08:40:20 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"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>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"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

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.

Thanks,
Shaohua

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