linux-kernel - Re: PCID and TLB flushes (was: [GIT PULL] kdbus for 4.1-rc1)

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Date:	Tue, 28 Apr 2015 18:41:43 -0400
From:	Rik van Riel <riel@...hat.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andy Lutomirski <luto@...capital.net>,
	Dave Hansen <dave.hansen@...el.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, x86@...nel.org
Subject: Re: PCID and TLB flushes (was: [GIT PULL] kdbus for 4.1-rc1)

On 04/28/2015 06:15 PM, Kirill A. Shutemov wrote:
> On Tue, Apr 28, 2015 at 01:42:10PM -0700, Andy Lutomirski wrote:
>> At some point, I'd like to implement PCID on x86 (if no one beats me
>> to it, and this is a low priority for me), which will allow us to skip
>> expensive TLB flushes while context switching.  I have no idea whether
>> ARM can do something similar.
> 
> I talked with Dave about implementing PCID and he thinks that it will be
> net loss. TLB entries will live longer and it means we would need to trigger
> more IPIs to flash them out when we have to. Cost of IPIs will be higher
> than benifit from hot TLB after context switch.

I suspect that may depend on how you do the shootdown.

If, when receiving a TLB shootdown for a non-current PCID, we just flush
all the entries for that PCID and remove the CPU from the mm's
cpu_vm_mask_var, we will never receive more than one shootdown IPI for
a non-current mm, but we will still get the benefits of TLB longevity
when dealing with eg. pipe workloads where tasks take turns running on
the same CPU.

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