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

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Message-ID: <CA+55aFwMfUduX_5kNKjo=S8ShEBb_OE_D6o-jSMy53Sa0iZHAw@mail.gmail.com>
Date:	Mon, 5 Dec 2011 15:26:12 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	torvalds@...ux-foundation.org, shaohua.li@...el.com,
	akpm@...ux-foundation.org, tglx@...utronix.de, mingo@...e.hu
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/mm] x86/mm: Avoid superflous leave_mm() in the TLB flush path

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.

                     Linus
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