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Message-ID: <1496776285.20270.64.camel@redhat.com>
Subject: Re: [RFC 05/11] x86/mm: Rework lazy TLB mode and TLB freshness
 tracking
From: Rik van Riel <riel@redhat.com>
Date: Tue, 06 Jun 2017 15:11:25 -0400
In-Reply-To: <9b939d6218b78352b9f13594ebf97c1c88a6c33d.1496701658.git.luto@kernel.org>
References: <cover.1496701658.git.luto@kernel.org>
	 <9b939d6218b78352b9f13594ebf97c1c88a6c33d.1496701658.git.luto@kernel.org>
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To: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>, Linus Torvalds <torvalds@linux-foundation.org>, Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mgorman@suse.de>, "linux-mm@kvack.org" <linux-mm@kvack.org>, Nadav Amit <nadav.amit@gmail.com>, Andrew Banman <abanman@sgi.com>, Mike Travis <travis@sgi.com>, Dimitri Sivanich <sivanich@sgi.com>, Juergen Gross <jgross@suse.com>, Boris Ostrovsky <boris.ostrovsky@oracle.com>


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On Mon, 2017-06-05 at 15:36 -0700, Andy Lutomirski wrote:

> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -122,8 +122,10 @@ static inline void switch_ldt(struct mm_struct
> *prev, struct mm_struct *next)
> =C2=A0
> =C2=A0static inline void enter_lazy_tlb(struct mm_struct *mm, struct
> task_struct *tsk)
> =C2=A0{
> -	if (this_cpu_read(cpu_tlbstate.state) =3D=3D TLBSTATE_OK)
> -		this_cpu_write(cpu_tlbstate.state, TLBSTATE_LAZY);
> +	int cpu =3D smp_processor_id();
> +
> +	if (cpumask_test_cpu(cpu, mm_cpumask(mm)))
> +		cpumask_clear_cpu(cpu, mm_cpumask(mm));
> =C2=A0}

This is an atomic write to a shared cacheline,
every time a CPU goes idle.

I am not sure you really want to do this, since
there are some workloads out there that have a
crazy number of threads, which go idle hundreds,
or even thousands of times a second, on dozens
of CPUs at a time. *cough*Java*cough*

Keeping track of the state in a CPU-local variable,
written with a non-atomic write, would be much more
CPU cache friendly here.

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