linux-kernel - Re: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier

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Date:   Tue, 17 Jul 2018 14:29:38 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Mike Galbraith <efault@....de>,
        kernel-team <kernel-team@...com>, Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier

On Tue, Jul 17, 2018 at 1:16 PM, Rik van Riel <riel@...riel.com> wrote:
> Can I skip both the cr4 and let switches when the TLB contents
> are no longer valid and got reloaded?
>
> If the TLB contents are still valid, either because we never went
> into lazy TLB mode, or because no invalidates happened while
> we were lazy, we immediately return.
>
> The cr4 and ldt reloads only happen if the TLB was invalidated
> while we were in lazy TLB mode.

Yes, since the only events that would change the LDT or the required
CR4 value will unconditionally broadcast to every CPU in mm_cpumask
regardless of whether they're lazy.  The interesting case is that you
go lazy, you miss an invalidation IPI because you were lazy, then you
go unlazy, notice the tlb_gen change, and flush.  If this happens, you
know that you only missed a page table update and not an LDT update or
a CR4 update, because the latter would have sent the IPI even though
you were lazy.  So you should skip the CR4 and LDT updates.

I suppose a different approach would be to fix the issue below and to
try to track when the LDT actually needs reloading.  But that latter
part seems a bit complicated for minimal gain.

(Do you believe me?  If not, please argue back!)

>> Hmm.  load_mm_cr4() should bypass itself when mm == &init_mm.  Want to
>> fix that part or should I?
>
> I would be happy to send in a patch for this, and one for
> the above optimization you pointed out.
>

Yes please!