linux-kernel - Re: [PATCH 7/7] x86/mm/tlb: Make lazy TLB mode lazier

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Message-ID: <20181001160737.GL3439@hirez.programming.kicks-ass.net>
Date:   Mon, 1 Oct 2018 18:07:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        will.deacon@....com, songliubraving@...com, mingo@...nel.org,
        luto@...nel.org, hpa@...or.com, npiggin@...il.com
Subject: Re: [PATCH 7/7] x86/mm/tlb: Make lazy TLB mode lazier

On Tue, Sep 25, 2018 at 11:58:44PM -0400, Rik van Riel wrote:
> @@ -594,8 +628,23 @@ void native_flush_tlb_others(const struct cpumask *cpumask,
>  					       (void *)info, 1);
>  		return;
>  	}
> -	smp_call_function_many(cpumask, flush_tlb_func_remote,
> +
> +	/*
> +	 * If no page tables were freed, we can skip sending IPIs to
> +	 * CPUs in lazy TLB mode. They will flush the CPU themselves
> +	 * at the next context switch.
> +	 *
> +	 * However, if page tables are getting freed, we need to send the
> +	 * IPI everywhere, to prevent CPUs in lazy TLB mode from tripping
> +	 * up on the new contents of what used to be page tables, while
> +	 * doing a speculative memory access.
> +	 */
> +	if (info->freed_tables)
> +		smp_call_function_many(cpumask, flush_tlb_func_remote,
>  			       (void *)info, 1);
> +	else
> +		on_each_cpu_cond_mask(tlb_is_not_lazy, flush_tlb_func_remote,
> +				(void *)info, 1, GFP_ATOMIC, cpumask);
>  }

And this is safe vs paravirt, because for native we now do _less_
invalidations.

That might warrant a mention in the Changelog perhaps.