From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Nicholas Piggin <npiggin@gmail.com>,
Anton Blanchard <anton@ozlabs.org>, Arnd Bergmann <arnd@arndb.de>,
Jens Axboe <axboe@kernel.dk>,
linux-arch <linux-arch@vger.kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
Andy Lutomirski <luto@amacapital.net>,
Andy Lutomirski <luto@kernel.org>, x86 <x86@kernel.org>
Subject: Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode
Date: Tue, 21 Jul 2020 11:15:13 -0400 (EDT) [thread overview]
Message-ID: <616209816.22376.1595344513051.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20200721150656.GN119549@hirez.programming.kicks-ass.net>
----- On Jul 21, 2020, at 11:06 AM, Peter Zijlstra peterz@infradead.org wrote:
> On Tue, Jul 21, 2020 at 08:04:27PM +1000, Nicholas Piggin wrote:
>
>> That being said, the x86 sync core gap that I imagined could be fixed
>> by changing to rq->curr == rq->idle test does not actually exist because
>> the global membarrier does not have a sync core option. So fixing the
>> exit_lazy_tlb points that this series does *should* fix that. So
>> PF_KTHREAD may be less problematic than I thought from implementation
>> point of view, only semantics.
>
> So I've been trying to figure out where that PF_KTHREAD comes from,
> commit 227a4aadc75b ("sched/membarrier: Fix p->mm->membarrier_state racy
> load") changed 'p->mm' to '!(p->flags & PF_KTHREAD)'.
>
> So the first version:
>
> https://lkml.kernel.org/r/20190906031300.1647-5-mathieu.desnoyers@efficios.com
>
> appears to unconditionally send the IPI and checks p->mm in the IPI
> context, but then v2:
>
> https://lkml.kernel.org/r/20190908134909.12389-1-mathieu.desnoyers@efficios.com
>
> has the current code. But I've been unable to find the reason the
> 'p->mm' test changed into '!(p->flags & PF_KTHREAD)'.
Looking back at my inbox, it seems like you are the one who proposed to
skip all kthreads:
https://lkml.kernel.org/r/20190904124333.GQ2332@hirez.programming.kicks-ass.net
>
> The comment doesn't really help either; sure we have the whole lazy mm
> thing, but that's ->active_mm, not ->mm.
>
> Possibly it is because {,un}use_mm() do not have sufficient barriers to
> make the remote p->mm test work? Or were we over-eager with the !p->mm
> doesn't imply kthread 'cleanups' at the time?
The nice thing about adding back kthreads to the threads considered for membarrier
IPI is that it has no observable effect on the user-space ABI. No pre-existing kthread
rely on this, and we just provide an additional guarantee for future kthread
implementations.
> Also, I just realized, I still have a fix for use_mm() now
> kthread_use_mm() that seems to have been lost.
I suspect we need to at least document the memory barriers in kthread_use_mm and
kthread_unuse_mm to state that they are required by membarrier if we want to
ipi kthreads as well.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2020-07-21 15:15 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-10 1:56 [RFC PATCH 0/7] mmu context cleanup, lazy tlb cleanup, Nicholas Piggin
2020-07-10 1:56 ` [RFC PATCH 1/7] asm-generic: add generic MMU versions of mmu context functions Nicholas Piggin
2020-07-10 1:56 ` [RFC PATCH 2/7] arch: use asm-generic mmu context for no-op implementations Nicholas Piggin
2020-07-10 1:56 ` [RFC PATCH 3/7] mm: introduce exit_lazy_tlb Nicholas Piggin
2020-07-10 1:56 ` [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode Nicholas Piggin
2020-07-10 9:42 ` Peter Zijlstra
2020-07-10 14:02 ` Mathieu Desnoyers
2020-07-10 17:04 ` Andy Lutomirski
2020-07-13 4:45 ` Nicholas Piggin
2020-07-13 13:47 ` Nicholas Piggin
2020-07-13 14:13 ` Mathieu Desnoyers
2020-07-13 15:48 ` Andy Lutomirski
2020-07-13 16:37 ` Nicholas Piggin
2020-07-16 4:15 ` Nicholas Piggin
2020-07-16 4:42 ` Nicholas Piggin
2020-07-16 15:46 ` Mathieu Desnoyers
2020-07-16 16:03 ` Mathieu Desnoyers
2020-07-16 18:58 ` Mathieu Desnoyers
2020-07-16 21:24 ` Alan Stern
2020-07-17 13:39 ` Mathieu Desnoyers
2020-07-17 14:51 ` Alan Stern
2020-07-17 15:39 ` Mathieu Desnoyers
2020-07-17 16:11 ` Alan Stern
2020-07-17 16:22 ` Mathieu Desnoyers
2020-07-17 17:44 ` Alan Stern
2020-07-17 17:52 ` Mathieu Desnoyers
2020-07-17 0:00 ` Nicholas Piggin
2020-07-16 5:18 ` Andy Lutomirski
2020-07-16 6:06 ` Nicholas Piggin
2020-07-16 8:50 ` Peter Zijlstra
2020-07-16 10:03 ` Nicholas Piggin
2020-07-16 11:00 ` peterz
2020-07-16 15:34 ` Mathieu Desnoyers
2020-07-16 23:26 ` Nicholas Piggin
2020-07-17 13:42 ` Mathieu Desnoyers
2020-07-20 3:03 ` Nicholas Piggin
2020-07-20 16:46 ` Mathieu Desnoyers
2020-07-21 10:04 ` Nicholas Piggin
2020-07-21 13:11 ` Mathieu Desnoyers
2020-07-21 14:30 ` Nicholas Piggin
2020-07-21 15:06 ` peterz
2020-07-21 15:15 ` Mathieu Desnoyers [this message]
2020-07-21 15:19 ` Peter Zijlstra
2020-07-21 15:22 ` Mathieu Desnoyers
2020-07-10 1:56 ` [RFC PATCH 5/7] lazy tlb: introduce lazy mm refcount helper functions Nicholas Piggin
2020-07-10 9:48 ` Peter Zijlstra
2020-07-10 1:56 ` [RFC PATCH 6/7] lazy tlb: allow lazy tlb mm switching to be configurable Nicholas Piggin
2020-07-10 1:56 ` [RFC PATCH 7/7] lazy tlb: shoot lazies, a non-refcounting lazy tlb option Nicholas Piggin
2020-07-10 9:35 ` Peter Zijlstra
2020-07-13 4:58 ` Nicholas Piggin
2020-07-13 15:59 ` Andy Lutomirski
2020-07-13 16:48 ` Nicholas Piggin
2020-07-13 18:18 ` Andy Lutomirski
2020-07-14 5:04 ` Nicholas Piggin
2020-07-14 6:31 ` Nicholas Piggin
2020-07-14 12:46 ` Andy Lutomirski
2020-07-14 13:23 ` Peter Zijlstra
2020-07-16 2:26 ` Nicholas Piggin
2020-07-16 2:35 ` Nicholas Piggin
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