linux-kernel - Re: [PATCH 6/8] lazy tlb: shoot lazies, a non-refcounting lazy tlb option

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Date:   Wed, 2 Dec 2020 17:29:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Nicholas Piggin <npiggin@...il.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
        Anton Blanchard <anton@...abs.org>
Subject: Re: [PATCH 6/8] lazy tlb: shoot lazies, a non-refcounting lazy tlb
 option

On Wed, Dec 02, 2020 at 06:38:12AM -0800, Andy Lutomirski wrote:
> 
> > On Dec 2, 2020, at 6:20 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > On Sun, Nov 29, 2020 at 02:01:39AM +1000, Nicholas Piggin wrote:
> >> +         * - A delayed freeing and RCU-like quiescing sequence based on
> >> +         *   mm switching to avoid IPIs completely.
> > 
> > That one's interesting too. so basically you want to count switch_mm()
> > invocations on each CPU. Then, periodically snapshot the counter on each
> > CPU, and when they've all changed, increment a global counter.
> > 
> > Then, you snapshot the global counter and wait for it to increment
> > (twice I think, the first increment might already be in progress).
> > 
> > The only question here is what should drive this machinery.. the tick
> > probably.
> > 
> > This shouldn't be too hard to do I think.
> > 
> > Something a little like so perhaps?
> 
> I don’t think this will work.  A CPU can go idle with lazy mm and nohz
> forever.  This could lead to unbounded memory use on a lightly loaded
> system.

Hurm.. quite so indeed. Fixing that seems to end up with requiring that
other proposal, such that we can tell which CPU has what active_mm
stuck.

Also, more complicated... :/