linux-kernel - Re: [PATCH 2/8] x86/mm: Handle unlazying membarrier core sync in the arch code

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Date:   Wed, 16 Jun 2021 13:49:12 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86 <x86@...nel.org>, Dave Hansen <dave.hansen@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/8] x86/mm: Handle unlazying membarrier core sync in
 the arch code

----- On Jun 15, 2021, at 11:21 PM, Andy Lutomirski luto@...nel.org wrote:
[...]
> @@ -473,16 +474,24 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct
> mm_struct *next,

[...]

> @@ -510,16 +520,35 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct
> mm_struct *next,
> 		 * If the TLB is up to date, just use it.
> 		 * The barrier synchronizes with the tlb_gen increment in
> 		 * the TLB shootdown code.
> +		 *
> +		 * As a future optimization opportunity, it's plausible
> +		 * that the x86 memory model is strong enough that this
> +		 * smp_mb() isn't needed.
> 		 */
> 		smp_mb();
> 		next_tlb_gen = atomic64_read(&next->context.tlb_gen);
> 		if (this_cpu_read(cpu_tlbstate.ctxs[prev_asid].tlb_gen) ==
> -				next_tlb_gen)
> +		    next_tlb_gen) {
> +#ifdef CONFIG_MEMBARRIER
> +			/*
> +			 * We switched logical mm but we're not going to
> +			 * write to CR3.  We already did smp_mb() above,
> +			 * but membarrier() might require a sync_core()
> +			 * as well.
> +			 */
> +			if (unlikely(atomic_read(&next->membarrier_state) &
> +				     MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE))
> +				sync_core_before_usermode();
> +#endif
> +
> 			return;
> +		}

[...]

I find that mixing up preprocessor #ifdef and code logic hurts readability.
Can you lift this into a static function within the same compile unit, and
provides an empty implementation for the #else case ?

Thanks,

Mathieu

	prev->sched_class->task_dead(prev);



-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com