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

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Message-ID: <20201130092628.GL2414@hirez.programming.kicks-ass.net>
Date:   Mon, 30 Nov 2020 10:26:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux-MM <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 Sat, Nov 28, 2020 at 07:54:57PM -0800, Andy Lutomirski wrote:
> Version (b) seems fairly straightforward to implement -- add RCU
> protection and a atomic_t special_ref_cleared (initially 0) to struct
> mm_struct itself.  After anyone clears a bit to mm_cpumask (which is
> already a barrier),

No it isn't. clear_bit() implies no barrier what so ever. That's x86
you're thinking about.

> they read mm_users.  If it's zero, then they scan
> mm_cpumask and see if it's empty.  If it is, they atomically swap
> special_ref_cleared to 1.  If it was zero before the swap, they do
> mmdrop().  I can imagine some tweaks that could make this a big
> faster, at least in the limit of a huge number of CPUs.