Re: [RFC v2 2/2] [MOCKUP] sched/mm: Lightweight lazy mm refcounting

From: Nicholas Piggin <npiggin@gmail.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: 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>,
	Andy Lutomirski <luto@kernel.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: Sat, 05 Dec 2020 14:49:04 +1000	[thread overview]
Message-ID: <1607141044.0ibmnpwoeq.astroid@bobo.none> (raw)
In-Reply-To: <D9715BFE-744E-49B4-A10B-32735123BE6D@amacapital.net>

Excerpts from Andy Lutomirski's message of December 5, 2020 12:37 am:
> 
> 
>> 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.

I'd go the other way. The mm_cpumark, TLB, and lazy maintainence is 
different between architectures anyway. I'd keep the simple refcount,
and the pretty simple shoot-lazies approaches for now at least until
a bit more is done on other fronts. If x86 is shooting down lazies on 
the final TLB flush as well, then I might be inclined to think that's
the better way to go in the long term. Shoot-lazies would be a bit of
a bolted on hack for powerpc/hash but it has ~zero impact to core code
really.

Thanks,
Nick