linux-kernel - Re: [PATCH 3/6] x86,tlb: make lazy TLB mode lazier

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Date:   Thu, 28 Jun 2018 16:05:18 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        kernel-team <kernel-team@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mike Galbraith <efault@....de>, songliubraving@...com
Subject: Re: [PATCH 3/6] x86,tlb: make lazy TLB mode lazier

On Wed, 2018-06-27 at 11:10 -0700, Andy Lutomirski wrote:
> 
> You left this comment:
> 
>                 /*
>                  * We don't currently support having a real mm loaded
> without
>                  * our cpu set in mm_cpumask().  We have all the
> bookkeeping
>                  * in place to figure out whether we would need to
> flush
>                  * if our cpu were cleared in mm_cpumask(), but we
> don't
>                  * currently use it.
>                  */
> 
> Presumably you should either clear the cpu from mm_cpumask when lazy
> or you shoudl update the comment.

The lazy TLB mode leaves the mm loaded, AND the
cpu set in mm_cpumask(). However, I guess while
the comment is technically accurate, it is no longer
relevant, so I will update it :)

> > +               /*
> > +                * Switching straight from one thread in a process
> > to another
> > +                * thread in the same process requires no TLB flush
> > at all.
> > +                */
> > +               if (!was_lazy)
> > +                       return;
> 
> Comment doesn't match code.  Maybe add "... if we weren't lazy"?

Done.

> > +
> > +               /*
> > +                * The code below checks whether there was a TLB
> > flush while
> > +                * this CPU was in lazy TLB mode. The barrier
> > ensures ordering
> > +                * with the TLB invalidation code advancing
> > .tlb_gen.
> > +                */
> > +               smp_rmb();
> 
> I think it may need to be smp_mb().  You're trying to order
> this_cpu_write() against subsequent reads.

I have updated the barrier to an smp_mb().

> In general, the changes to this function are very hard to review
> because you're mixing semantic changes and restructuring the
> function.
> Is there any way you could avoid that?  Or maybe just open-code a
> tlb_gen check in the unlazying path?
> 
> 
> > +       /*
> > +        * Instead of sending IPIs to CPUs in lazy TLB mode, move
> > that
> > +        * CPU's TLB state to TLBSTATE_FLUSH, causing the TLB to be
> > flushed
> > +        * at the next context switch, or at page table free time.
> > +        */
> 
> Stale comment?

Will fix.

I am running some last tests now, and will send
v3 soon.

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