linux-kernel - Re: [RFC PATCH 2/3] sched: membarrier: cover kthread_use

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Message-ID: <20201007160820.GK2628@hirez.programming.kicks-ass.net>
Date:   Wed, 7 Oct 2020 18:08:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>, paulmck <paulmck@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 2/3] sched: membarrier: cover kthread_use_mm (v3)

On Wed, Oct 07, 2020 at 11:39:30AM -0400, Mathieu Desnoyers wrote:
> Moving the membarrier_switch_mm to cover kthread cases was to ensure (2), but if we
> add a p->mm NULL check in the global expedited iteration, I think we would be OK
> leaving the stale runqueue's membarrier state while in lazy tlb state.
> 
> As far as (1) is concerned, I think your idea would work, because as you say we will
> have the proper barriers in kthread use/unuse mm.
> 
> I just wonder whether having this stale membarrier state for lazy tlb is warranted
> performance-wise, as it adds complexity: the rq membarrier state will therefore not be
> relevant when we are in lazy tlb mode.
> 
> Thoughts ?

Well, the way I got here was that I considered the membarrier state
update tied to switch_mm(), and in that regard my proposal is a
simplification.