linux-kernel - Re: [PATCH 2/4] mm: Send one IPI per CPU to TLB flush all entries after unmapping pages

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Date:	Tue, 09 Jun 2015 16:01:46 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	Hugh Dickins <hughd@...gle.com>, Minchan Kim <minchan@...nel.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	H Peter Anvin <hpa@...or.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] mm: Send one IPI per CPU to TLB flush all entries
 after unmapping pages

On 06/09/2015 01:31 PM, Mel Gorman wrote:
> An IPI is sent to flush remote TLBs when a page is unmapped that was
> potentially accesssed by other CPUs. There are many circumstances where
> this happens but the obvious one is kswapd reclaiming pages belonging to
> a running process as kswapd and the task are likely running on separate CPUs.

> It's still a noticeable improvement with vmstat showing interrupts went
> from roughly 500K per second to 45K per second.
> 
> The patch will have no impact on workloads with no memory pressure or
> have relatively few mapped pages. It will have an unpredictable impact
> on the workload running on the CPU being flushed as it'll depend on how
> many TLB entries need to be refilled and how long that takes. Worst case,
> the TLB will be completely cleared of active entries when the target PFNs
> were not resident at all.
> 
> Signed-off-by: Mel Gorman <mgorman@...e.de>

Reviewed-by: Rik van Riel <riel@...hat.com>


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