Re: [PATCH 0/2] arm64: Introduce boot parameter to disable TLB flush instruction within the same inner shareable domain

[Jon Masters]

On 11/1/19 1:28 PM, Will Deacon wrote:

> On Fri, Nov 01, 2019 at 09:56:05AM +0000, qi.fuli@xxxxxxxxxxx wrote:

> > In this thread, I explained that:
> > * I found a performance problem which is caused by TLBI-is instruction.
> > * The problem occurs like this:
> >    1) On a core, OS tries to flush TLB using TLBI-is instruction
> >    2) TLBI-is instruction causes a broadcast to all other cores, and
> >    each core received hard-wired signal
> >    3) Each core check if there are TLB entries which have the specified
> > ASID/VA

(the above confuses implementation with architecture)

<snip>

> I think it's worth bearing in mind that I have little sympathy for the
> problem that you are seeing. As far as I can tell, you've done the
> following:
>
>    1. You designed a CPU micro-architecture that stalls whenever it receives
>       a TLB invalidation request.

s/SPARC/Arm/ && wire in DVM

>    2. You integrated said CPU design into a system where broadcast TLB
>       invalidation is not filtered and therefore stalls every CPU every
>       time that /any/ TLB invalidation is broadcast.
>
>    3. You deployed a mixture of Linux and jitter-sensitive software on
>       this system, and now you're failing to meet your performance
>       requirements.
>
> Have I got that right?
>
> If so, given that your CPU design isn't widely available, nobody else
> appears to have made this mistake and jitter hasn't been reported as an
> issue for any other systems, it's very unlikely that we're going to make
> invasive upstream kernel changes to support you. I'm sorry, but all I can
> suggest is that you check that your micro-architecture and performance
> requirements are aligned with the design of Linux *before* building another
> machine like this in future.
>
> I hate to be blunt, but I also don't want to waste your time.

I always tried to ask nicely for the above to be heeded. There's a 
difference between "hi, our implementation doesn't scale, and here's 
why" vs "there's a problem with all TLBIs...". There isn't. The problem 
is the implementation and that should have been called out first thing.

Jon.

--
Computer Architect