linux-kernel - Re: [PATCH] mm: Add new vma flag VM_LOCAL

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Date:   Mon, 14 May 2018 17:44:06 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Boaz Harrosh <boazh@...app.com>, Jeff Moyer <jmoyer@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Rik van Riel <riel@...hat.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Amit Golander <Amit.Golander@...app.com>
Subject: Re: [PATCH] mm: Add new vma flag VM_LOCAL_CPU

On Mon, May 14, 2018 at 02:49:01PM -0700, Andrew Morton wrote:
> On Mon, 14 May 2018 20:28:01 +0300 Boaz Harrosh <boazh@...app.com> wrote:
> > In this project we utilize a per-core server thread so everything
> > is kept local. If we use the regular zap_ptes() API All CPU's
> > are scheduled for the unmap, though in our case we know that we
> > have only used a single core. The regular zap_ptes adds a very big
> > latency on every operation and mostly kills the concurrency of the
> > over all system. Because it imposes a serialization between all cores
> 
> I'd have thought that in this situation, only the local CPU's bit is
> set in the vma's mm_cpumask() and the remote invalidations are not
> performed.  Is that a misunderstanding, or is all that stuff not working
> correctly?

I think you misunderstand Boaz's architecture.  He has one thread per CPU,
so every bit will be set in the mm's (not vma's) mm_cpumask.