From: Andy Lutomirski <luto@amacapital.net>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: Andy Lutomirski <luto@kernel.org>,
Anton Blanchard <anton@ozlabs.org>, Arnd Bergmann <arnd@arndb.de>,
Catalin Marinas <catalin.marinas@arm.com>,
Dave Hansen <dave.hansen@intel.com>, Jann Horn <jannh@google.com>,
linux-arch <linux-arch@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Nadav Amit <nadav.amit@gmail.com>,
Rik van Riel <riel@surriel.com>, Will Deacon <will@kernel.org>,
X86 ML <x86@kernel.org>
Subject: Re: [RFC v2 2/2] [MOCKUP] sched/mm: Lightweight lazy mm refcounting
Date: Fri, 4 Dec 2020 06:37:04 -0800 [thread overview]
Message-ID: <D9715BFE-744E-49B4-A10B-32735123BE6D@amacapital.net> (raw)
In-Reply-To: <1607065599.ecww2w3xq3.astroid@bobo.none>
> On Dec 3, 2020, at 11:54 PM, Nicholas Piggin <npiggin@gmail.com> wrote:
>
> Excerpts from Andy Lutomirski's message of December 4, 2020 3:26 pm:
>> This is a mockup. It's designed to illustrate the algorithm and how the
>> code might be structured. There are several things blatantly wrong with
>> it:
>>
>> The coding stype is not up to kernel standards. I have prototypes in the
>> wrong places and other hacks.
>>
>> There's a problem with mm_cpumask() not being reliable.
>
> Interesting, this might be a way to reduce those IPIs with fairly
> minimal fast path cost. Would be interesting to see how much performance
> advantage it has over my dumb simple shoot-lazies.
My real motivation isn’t really performance per se. I think there’s considerable value in keeping the core algorithms the same across all architectures, and I think my approach can manage that with only a single hint from the architecture as to which CPUs to scan.
With shoot-lazies, in contrast, enabling it everywhere would either malfunction or have very poor performance or even DoS issues on arches like arm64 and s390x that don’t track mm_cpumask at all. I’m sure we could come up with some way to mitigate that, but I think that my approach may be better overall for keeping the core code uniform and relatively straightforward.
>
> For powerpc I don't think we'd be inclined to go that way, so don't feel
> the need to add this complexity for us alone -- we'd be more inclined to
> move the exit lazy to the final TLB shootdown path, which we're slowly
> getting more infrastructure in place to do.
>
>
> There's a few nits but I don't think I can see a fundamental problem
> yet.
Thanks!
I can polish the patch, but I want to be sure the memory ordering parts are clear.
>
> Thanks,
> Nick
next prev parent reply other threads:[~2020-12-04 14:38 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-04 5:26 [RFC v2 0/2] lazy mm refcounting Andy Lutomirski
2020-12-04 5:26 ` [RFC v2 1/2] [NEEDS HELP] x86/mm: Handle unlazying membarrier core sync in the arch code Andy Lutomirski
2020-12-04 7:06 ` Nicholas Piggin
2020-12-04 8:17 ` Nadav Amit
2020-12-04 20:39 ` Mathieu Desnoyers
2020-12-04 20:24 ` Mathieu Desnoyers
2020-12-04 5:26 ` [RFC v2 2/2] [MOCKUP] sched/mm: Lightweight lazy mm refcounting Andy Lutomirski
2020-12-04 7:54 ` Nicholas Piggin
2020-12-04 14:37 ` Andy Lutomirski [this message]
2020-12-05 4:49 ` Nicholas Piggin
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