From: Andy Lutomirski <luto@kernel.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>,
LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Arnd Bergmann <arnd@arndb.de>,
Peter Zijlstra <peterz@infradead.org>,
linux-arch <linux-arch@vger.kernel.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
Linux-MM <linux-mm@kvack.org>,
Anton Blanchard <anton@ozlabs.org>
Subject: Re: [PATCH 6/8] lazy tlb: shoot lazies, a non-refcounting lazy tlb option
Date: Sun, 29 Nov 2020 12:16:26 -0800 [thread overview]
Message-ID: <CALCETrWBtCfD+jZ3S+O8FK-HFPODuhbDEbbfWvS=-iPATNFAOA@mail.gmail.com> (raw)
In-Reply-To: <CALCETrVXUbe8LfNn-Qs+DzrOQaiw+sFUg1J047yByV31SaTOZw@mail.gmail.com>
On Sat, Nov 28, 2020 at 7:54 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> On Sat, Nov 28, 2020 at 8:02 AM Nicholas Piggin <npiggin@gmail.com> wrote:
> >
> > On big systems, the mm refcount can become highly contented when doing
> > a lot of context switching with threaded applications (particularly
> > switching between the idle thread and an application thread).
> >
> > Abandoning lazy tlb slows switching down quite a bit in the important
> > user->idle->user cases, so so instead implement a non-refcounted scheme
> > that causes __mmdrop() to IPI all CPUs in the mm_cpumask and shoot down
> > any remaining lazy ones.
> >
> > Shootdown IPIs are some concern, but they have not been observed to be
> > a big problem with this scheme (the powerpc implementation generated
> > 314 additional interrupts on a 144 CPU system during a kernel compile).
> > There are a number of strategies that could be employed to reduce IPIs
> > if they turn out to be a problem for some workload.
>
> I'm still wondering whether we can do even better.
>
Hold on a sec.. __mmput() unmaps VMAs, frees pagetables, and flushes
the TLB. On x86, this will shoot down all lazies as long as even a
single pagetable was freed. (Or at least it will if we don't have a
serious bug, but the code seems okay. We'll hit pmd_free_tlb, which
sets tlb->freed_tables, which will trigger the IPI.) So, on
architectures like x86, the shootdown approach should be free. The
only way it ought to have any excess IPIs is if we have CPUs in
mm_cpumask() that don't need IPI to free pagetables, which could
happen on paravirt.
Can you try to figure out why you saw any increase in IPIs? It would
be nice if we can make the new code unconditional.
next prev parent reply other threads:[~2020-11-29 20:16 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-28 16:01 [PATCH 0/8] shoot lazy tlbs Nicholas Piggin
2020-11-28 16:01 ` [PATCH 1/8] lazy tlb: introduce exit_lazy_tlb Nicholas Piggin
2020-11-29 0:38 ` Andy Lutomirski
2020-12-02 2:49 ` Nicholas Piggin
2020-11-28 16:01 ` [PATCH 2/8] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode Nicholas Piggin
2020-11-28 17:55 ` Andy Lutomirski
2020-12-02 2:49 ` Nicholas Piggin
2020-12-03 5:09 ` Andy Lutomirski
2020-12-05 8:00 ` Nicholas Piggin
2020-12-05 16:11 ` Andy Lutomirski
2020-12-05 23:14 ` Nicholas Piggin
2020-12-06 0:36 ` Andy Lutomirski
2020-12-06 3:59 ` Nicholas Piggin
2020-12-11 0:11 ` Andy Lutomirski
2020-12-14 4:07 ` Nicholas Piggin
2020-12-14 5:53 ` Nicholas Piggin
2020-11-30 14:57 ` Mathieu Desnoyers
2020-11-28 16:01 ` [PATCH 3/8] x86: remove ARCH_HAS_SYNC_CORE_BEFORE_USERMODE Nicholas Piggin
2020-11-28 16:01 ` [PATCH 4/8] lazy tlb: introduce lazy mm refcount helper functions Nicholas Piggin
2020-11-28 16:01 ` [PATCH 5/8] lazy tlb: allow lazy tlb mm switching to be configurable Nicholas Piggin
2020-11-29 0:36 ` Andy Lutomirski
2020-12-02 2:49 ` Nicholas Piggin
2020-11-28 16:01 ` [PATCH 6/8] lazy tlb: shoot lazies, a non-refcounting lazy tlb option Nicholas Piggin
2020-11-29 3:54 ` Andy Lutomirski
2020-11-29 20:16 ` Andy Lutomirski [this message]
2020-11-30 9:25 ` Peter Zijlstra
2020-11-30 18:31 ` Andy Lutomirski
2020-12-01 21:27 ` Will Deacon
2020-12-01 21:50 ` Andy Lutomirski
2020-12-01 23:04 ` Will Deacon
2020-12-02 3:47 ` Nicholas Piggin
2020-12-03 5:05 ` Andy Lutomirski
2020-12-03 17:03 ` Alexander Gordeev
2020-12-03 17:14 ` Andy Lutomirski
2020-12-03 18:33 ` Alexander Gordeev
2020-11-30 9:26 ` Peter Zijlstra
2020-11-30 9:30 ` Peter Zijlstra
2020-11-30 9:34 ` Peter Zijlstra
2020-12-02 3:09 ` Nicholas Piggin
2020-12-02 11:17 ` Peter Zijlstra
2020-12-02 12:45 ` Peter Zijlstra
2020-12-02 14:19 ` Peter Zijlstra
2020-12-02 14:38 ` Andy Lutomirski
2020-12-02 16:29 ` Peter Zijlstra
2020-11-28 16:01 ` [PATCH 7/8] powerpc: use lazy mm refcount helper functions Nicholas Piggin
2020-11-28 16:01 ` [PATCH 8/8] powerpc/64s: enable MMU_LAZY_TLB_SHOOTDOWN Nicholas Piggin
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