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From: Nadav Amit <nadav.amit@gmail.com>
To: Rik van Riel <riel@surriel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux-MM <linux-mm@kvack.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	Anton Blanchard <anton@ozlabs.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@ozlabs.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	linux-arch <linux-arch@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [PATCH 16/23] sched: Use lightweight hazard pointers to grab lazy mms
Date: Sun, 9 Jan 2022 11:34:40 -0800	[thread overview]
Message-ID: <1B6896F0-7A51-4936-8B50-0B86551FA3B7@gmail.com> (raw)
In-Reply-To: <ba2522fbabc8a81befd27ef20588f9356f7b885b.camel@surriel.com>



> On Jan 9, 2022, at 11:22 AM, Rik van Riel <riel@surriel.com> wrote:
> 
> On Sun, 2022-01-09 at 00:49 -0800, Nadav Amit wrote:
>> 
>> It is possible for instance to get rid of is_lazy, keep the CPU
>> on mm_cpumask when switching to kernel thread, and then if/when
>> an IPI is received remove it from cpumask to avoid further
>> unnecessary TLB shootdown IPIs.
>> 
>> I do not know whether it is a pure win, but there is a tradeoff.
> 
> That's not a win at all. It is what we used to have before
> the lazy mm stuff was re-introduced, and it led to quite a
> few unnecessary IPIs, which are especially slow on virtual
> machines, where the target CPU may not be running.

You make a good point about VMs.

IIUC Lazy-TLB serves several goals:

1. Avoid arch address-space switch to save switching time and
   TLB misses.
2. Prevent unnecessary IPIs while kernel threads run.
3. Avoid cache-contention on mm_cpumask when switching to a kernel
   thread.

Your concern is with (2), and this one is easy to keep regardless
of the rest.

I am not sure that (3) is actually helpful, since it might lead
to more cache activity than without lazy-TLB, but that is somewhat
orthogonal to everything else.

As for (1), which is the most fragile aspect, unless you use
shadow page-tables, I am not sure there is a significant benefit.


  reply	other threads:[~2022-01-09 19:34 UTC|newest]

Thread overview: 79+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-08 16:43 [PATCH 00/23] mm, sched: Rework lazy mm handling Andy Lutomirski
2022-01-08 16:43 ` [PATCH 01/23] membarrier: Document why membarrier() works Andy Lutomirski
2022-01-12 15:30   ` Mathieu Desnoyers
2022-01-08 16:43 ` [PATCH 02/23] x86/mm: Handle unlazying membarrier core sync in the arch code Andy Lutomirski
2022-01-12 15:40   ` Mathieu Desnoyers
2022-01-08 16:43 ` [PATCH 03/23] membarrier: Remove membarrier_arch_switch_mm() prototype in core code Andy Lutomirski
2022-01-08 16:43 ` [PATCH 04/23] membarrier: Make the post-switch-mm barrier explicit Andy Lutomirski
2022-01-12 15:52   ` Mathieu Desnoyers
2022-01-08 16:43 ` [PATCH 05/23] membarrier, kthread: Use _ONCE accessors for task->mm Andy Lutomirski
2022-01-12 15:55   ` Mathieu Desnoyers
2022-01-08 16:43 ` [PATCH 06/23] powerpc/membarrier: Remove special barrier on mm switch Andy Lutomirski
2022-01-08 16:43   ` Andy Lutomirski
2022-01-10  8:42   ` Christophe Leroy
2022-01-10  8:42     ` Christophe Leroy
2022-01-12 15:57   ` Mathieu Desnoyers
2022-01-12 15:57     ` Mathieu Desnoyers
2022-01-08 16:43 ` [PATCH 07/23] membarrier: Rewrite sync_core_before_usermode() and improve documentation Andy Lutomirski
2022-01-08 16:43   ` Andy Lutomirski
2022-01-08 16:43   ` Andy Lutomirski
2022-01-12 16:11   ` Mathieu Desnoyers
2022-01-12 16:11     ` Mathieu Desnoyers
2022-01-12 16:11     ` Mathieu Desnoyers
2022-01-08 16:43 ` [PATCH 08/23] membarrier: Remove redundant clear of mm->membarrier_state in exec_mmap() Andy Lutomirski
2022-01-12 16:13   ` Mathieu Desnoyers
2022-01-08 16:43 ` [PATCH 09/23] membarrier: Fix incorrect barrier positions during exec and kthread_use_mm() Andy Lutomirski
2022-01-12 16:30   ` Mathieu Desnoyers
2022-01-12 17:08     ` Mathieu Desnoyers
2022-01-08 16:43 ` [PATCH 10/23] x86/events, x86/insn-eval: Remove incorrect active_mm references Andy Lutomirski
2022-01-08 16:43 ` [PATCH 11/23] sched/scs: Initialize shadow stack on idle thread bringup, not shutdown Andy Lutomirski
2022-01-10 22:06   ` Sami Tolvanen
2022-01-08 16:43 ` [PATCH 12/23] Rework "sched/core: Fix illegal RCU from offline CPUs" Andy Lutomirski
2022-01-08 16:43 ` [PATCH 13/23] exec: Remove unnecessary vmacache_seqnum clear in exec_mmap() Andy Lutomirski
2022-01-08 16:43 ` [PATCH 14/23] sched, exec: Factor current mm changes out from exec Andy Lutomirski
2022-01-08 16:44 ` [PATCH 15/23] kthread: Switch to __change_current_mm() Andy Lutomirski
2022-01-08 16:44 ` [PATCH 16/23] sched: Use lightweight hazard pointers to grab lazy mms Andy Lutomirski
2022-01-08 19:22   ` Linus Torvalds
2022-01-08 22:04     ` Andy Lutomirski
2022-01-09  0:27       ` Linus Torvalds
2022-01-09  0:53       ` Linus Torvalds
2022-01-09  3:58         ` Andy Lutomirski
2022-01-09  4:38           ` Linus Torvalds
2022-01-09 20:19             ` Andy Lutomirski
2022-01-09 20:48               ` Linus Torvalds
2022-01-09 21:51                 ` Linus Torvalds
2022-01-10  0:52                   ` Andy Lutomirski
2022-01-10  2:36                     ` Rik van Riel
2022-01-10  3:51                       ` Linus Torvalds
2022-01-10  4:56                   ` Nicholas Piggin
2022-01-10  5:17                     ` Nicholas Piggin
2022-01-10 17:19                       ` Linus Torvalds
2022-01-11  2:24                         ` Nicholas Piggin
2022-01-10 20:52                     ` Andy Lutomirski
2022-01-11  3:10                       ` Nicholas Piggin
2022-01-11 15:39                         ` Andy Lutomirski
2022-01-11 22:48                           ` Nicholas Piggin
2022-01-12  0:42                             ` Nicholas Piggin
2022-01-11 10:39                 ` Will Deacon
2022-01-11 15:22                   ` Andy Lutomirski
2022-01-09  5:56   ` Nadav Amit
2022-01-09  6:48     ` Linus Torvalds
2022-01-09  8:49       ` Nadav Amit
2022-01-09 19:10         ` Linus Torvalds
2022-01-09 19:52           ` Andy Lutomirski
2022-01-09 20:00             ` Linus Torvalds
2022-01-09 20:34             ` Nadav Amit
2022-01-09 20:48               ` Andy Lutomirski
2022-01-09 19:22         ` Rik van Riel
2022-01-09 19:34           ` Nadav Amit [this message]
2022-01-09 19:37             ` Rik van Riel
2022-01-09 19:51               ` Nadav Amit
2022-01-09 19:54                 ` Linus Torvalds
2022-01-08 16:44 ` [PATCH 17/23] x86/mm: Make use/unuse_temporary_mm() non-static Andy Lutomirski
2022-01-08 16:44 ` [PATCH 18/23] x86/mm: Allow temporary mms when IRQs are on Andy Lutomirski
2022-01-08 16:44 ` [PATCH 19/23] x86/efi: Make efi_enter/leave_mm use the temporary_mm machinery Andy Lutomirski
2022-01-10 13:13   ` Ard Biesheuvel
2022-01-08 16:44 ` [PATCH 20/23] x86/mm: Remove leave_mm() in favor of unlazy_mm_irqs_off() Andy Lutomirski
2022-01-08 16:44 ` [PATCH 21/23] x86/mm: Use unlazy_mm_irqs_off() in TLB flush IPIs Andy Lutomirski
2022-01-08 16:44 ` [PATCH 22/23] x86/mm: Optimize for_each_possible_lazymm_cpu() Andy Lutomirski
2022-01-08 16:44 ` [PATCH 23/23] x86/mm: Opt in to IRQs-off activate_mm() Andy Lutomirski

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