linux-kernel - Re: [PATCH 6/8] lazy tlb: shoot lazies, a non-refcounting lazy tlb option

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Date:   Thu, 3 Dec 2020 09:14:22 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Nicholas Piggin <npiggin@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux-MM <linux-mm@...ck.org>, Anton Blanchard <anton@...abs.org>
Subject: Re: [PATCH 6/8] lazy tlb: shoot lazies, a non-refcounting lazy tlb option



> On Dec 3, 2020, at 9:09 AM, Alexander Gordeev <agordeev@...ux.ibm.com> wrote:
> 
> On Mon, Nov 30, 2020 at 10:31:51AM -0800, Andy Lutomirski wrote:
>> other arch folk: there's some background here:

> 
>> 
>> power: Ridiculously complicated, seems to vary by system and kernel config.
>> 
>> So, Nick, your unconditional IPI scheme is apparently a big
>> improvement for power, and it should be an improvement and have low
>> cost for x86.  On arm64 and s390x it will add more IPIs on process
>> exit but reduce contention on context switching depending on how lazy
> 
> s390 does not invalidate TLBs per-CPU explicitly - we have special
> instructions for that. Those in turn initiate signalling to other
> CPUs, completely transparent to OS.

Just to make sure I understand: this means that you broadcast flushes to all CPUs, not just a subset?

> 
> Apart from mm_count, I am struggling to realize how the suggested
> scheme could change the the contention on s390 in connection with
> TLB. Could you clarify a bit here, please?

I’m just talking about mm_count. Maintaining mm_count is quite expensive on some workloads.

> 
>> TLB works.  I suppose we could try it for all architectures without
>> any further optimizations.  Or we could try one of the perhaps
>> excessively clever improvements I linked above.  arm64, s390x people,
>> what do you think?
> 
> I do not immediately see anything in the series that would harm
> performance on s390.
> 
> We however use mm_cpumask to distinguish between local and global TLB
> flushes. With this series it looks like mm_cpumask is *required* to
> be consistent with lazy users. And that is something quite diffucult
> for us to adhere (at least in the foreseeable future).

You don’t actually need to maintain mm_cpumask — we could scan all CPUs instead.

> 
> But actually keeping track of lazy users in a cpumask is something
> the generic code would rather do AFAICT.

The problem is that arches don’t agree on what the contents of mm_cpumask should be.  Tracking a mask of exactly what the arch wants in generic code is a nontrivial operation.