From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Nadav Amit <nadav.amit@gmail.com>,
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>,
"the arch/x86 maintainers" <x86@kernel.org>,
Rik van Riel <riel@surriel.com>,
Dave Hansen <dave.hansen@intel.com>,
"Peter Zijlstra (Intel)" <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 12:00:32 -0800 [thread overview]
Message-ID: <CAHk-=wiNTupLcOL7z=FNq2YoD=H=urRHFZQNqHAvJ+8R=ZyOVw@mail.gmail.com> (raw)
In-Reply-To: <355c148c-06a8-4e15-a77b-0ea2e22bf708@www.fastmail.com>
On Sun, Jan 9, 2022 at 11:53 AM Andy Lutomirski <luto@kernel.org> wrote:
>
> My original PCID series actually did remove lazy TLB on x86. I don't
> remember why, but people objected. The issue isn't the limited PCID
> space -- IIRC it's just that MOV CR3 is slooooow. If we get rid of
> lazy TLB on x86, then we are writing CR3 twice on even a very short
> idle. That adds maybe 1k cycles, which isn't great.
Yeah, my gut feel is that lazy-TLB almost certainly makes sense on x86.
And the grab/mmput overhead and associated cacheline ping-pong is (I
think) something we could just get rid of on x86 due to the IPI model.
There are probably other costs to lazy TLB, and I can imagine that
there are other maintenance costs, but yes, cr3 moves have always been
expensive on x86 even aside from the actual TLB switch.
But I could easily imagine the situation being different on arm64, for example.
But numbers beat "gut feel" and "easily imagine" every time. So it
would be kind of nice to have that ...
Linus
next prev parent reply other threads:[~2022-01-09 20:00 UTC|newest]
Thread overview: 67+ 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 06/23] powerpc/membarrier: Remove special barrier on mm switch Andy Lutomirski
2022-01-10 8:42 ` Christophe Leroy
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
2022-01-12 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
[not found] ` <1641790309.2vqc26hwm3.astroid@bobo.none>
[not found] ` <1641791321.kvkq0n8kbq.astroid@bobo.none>
2022-01-10 17:19 ` Linus Torvalds
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
2022-01-11 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 [this message]
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
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
[not found] ` <e6e7c11c38a3880e56fb7dfff4fa67090d831a3b.1641659630.git.luto@kernel.org>
2022-01-12 15:55 ` [PATCH 05/23] membarrier, kthread: Use _ONCE accessors for task->mm Mathieu Desnoyers
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