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

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Message-Id: <BA2FB4C0-55EA-481A-824C-95B94EA29FAB@amacapital.net>
Date:   Wed, 2 Dec 2020 06:38:12 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
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 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.