linux-kernel - Re: [RFC 00/10] x86 TLB flush cleanups, moving toward PCID support

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Date:   Mon, 8 May 2017 09:36:47 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [RFC 00/10] x86 TLB flush cleanups, moving toward PCID support


> On May 7, 2017, at 5:38 AM, Andy Lutomirski <luto@...nel.org> wrote:
> 
> As I've been working on polishing my PCID code, a major problem I've
> encountered is that there are too many x86 TLB flushing code paths and
> that they have too many inconsequential differences.  The result was
> that earlier versions of the PCID code were a colossal mess and very
> difficult to understand.
> 
> This series goes a long way toward cleaning up the mess.  With all the
> patches applied, there is a single function that contains the meat of
> the code to flush the TLB on a given CPU, and all the tlb flushing
> APIs call it for both local and remote CPUs.
> 
> This series should only adversely affect the kernel in a couple of
> minor ways:
> 
> - It makes smp_mb() unconditional when flushing TLBs.  We used to
>   use the TLB flush itself to mostly avoid smp_mb() on the initiating
>   CPU.
> 
> - On UP kernels, we lose the dubious optimization of inlining nerfed
>   variants of all the TLB flush APIs.  This bloats the kernel a tiny
>   bit, although it should increase performance, since the SMP
>   versions were better.
> 
> Patch 10 in here is a little bit off topic.  It's a cleanup that's
> also needed before PCID can go in, but it's not directly about
> TLB flushing.
> 
> Thoughts?

In general I like the changes. I needed to hack Linux TLB shootdowns for
a research project just because I could not handle the code otherwise.
I ended up doing some of changes that you have done.

I just have two general comments:

- You may want to consider merging the kernel mappings invalidation
  with the userspace mappings invalidations as well, since there are
  still code redundancies.

- Don’t expect too much from concurrent TLB invalidations. In many
  cases the IPI latency dominates the overhead from my experience.

Regards,
Nadav