linux-kernel - Re: [PATCH v3 11/11] x86/mm: Try to preserve old TLB entries using PCID

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Date:   Thu, 22 Jun 2017 23:22:29 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 11/11] x86/mm: Try to preserve old TLB entries using
 PCID

On Thu, 22 Jun 2017, Andy Lutomirski wrote:
> On Thu, Jun 22, 2017 at 5:21 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > Now one other optimization which should be trivial to add is to keep the 4
> > asid context entries in cpu_tlbstate and cache the last asid in thread
> > info. If that's still valid then use it otherwise unconditionally get a new
> > one. That avoids the whole loop machinery and thread info is cache hot in
> > the context switch anyway. Delta patch on top of your version below.
> 
> I'm not sure I understand.  If an mm has ASID 0 on CPU 0 and ASID 1 on
> CPU 1 and a thread in that mm bounces back and forth between those
> CPUs, won't your patch cause it to flush every time?

Yeah, I was too focussed on the non migratory case, where two tasks from
different processes play rapid ping pong. That's what I was looking at for
various reasons.

There the cached asid really helps by avoiding the loop completely, but
yes, the search needs to be done for the bouncing between CPUs case.

So maybe a combo of those might be interesting.

Thanks,

	tglx