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From: Andy Lutomirski <luto@kernel.org>
To: Rik van Riel <riel@surriel.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	kernel-team <kernel-team@fb.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Ingo Molnar <mingo@kernel.org>, Mike Galbraith <efault@gmx.de>,
	Dave Hansen <dave.hansen@intel.com>,
	will.daecon@arm.com, Catalin Marinas <catalin.marinas@arm.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [PATCH 10/10] mm,sched: conditionally skip lazy TLB mm refcounting
Date: Sat, 28 Jul 2018 21:21:17 -0700	[thread overview]
Message-ID: <CALCETrWS4pE5Tv3OErM3AD-E=CGG3U+2CBVecRF3N0c+g42vEg@mail.gmail.com> (raw)
In-Reply-To: <20180728215357.3249-11-riel@surriel.com>

On Sat, Jul 28, 2018 at 2:53 PM, Rik van Riel <riel@surriel.com> wrote:
> Conditionally skip lazy TLB mm refcounting. When an architecture has
> CONFIG_ARCH_NO_ACTIVE_MM_REFCOUNTING enabled, an mm that is used in
> lazy TLB mode anywhere will get shot down from exit_mmap, and there
> in no need to incur the cache line bouncing overhead of refcounting
> a lazy TLB mm.

Unless I've misunderstood something, this patch results in idle tasks
whose active_mm has been freed still having active_mm pointing at
freed memory.  This isn't strictly speaking a bug, but it's extremely
confusing and risks all kinds of nasty errors.  That's why I prefer
the approach of actually removing the active_mm field on x86 rather
than merely removing the refcount.

I realize that this will add more ifdeffery and make the patch a bit
bigger, but I think it'll be much more robust.  Not to mention that it
will save a pointer an a refcount per mm_struct, but that barely
matters.

  reply	other threads:[~2018-07-29  4:21 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-28 21:53 [PATCH 0/10] x86,tlb,mm: more lazy TLB cleanups & optimizations Rik van Riel
2018-07-28 21:53 ` [PATCH 01/10] x86,tlb: clarify memory barrier in switch_mm_irqs_off Rik van Riel
2018-07-29  2:59   ` Andy Lutomirski
2018-07-28 21:53 ` [PATCH 02/10] smp: use __cpumask_set_cpu in on_each_cpu_cond Rik van Riel
2018-07-29  2:59   ` Andy Lutomirski
2018-07-28 21:53 ` [PATCH 03/10] smp,cpumask: introduce on_each_cpu_cond_mask Rik van Riel
2018-07-29  2:57   ` Andy Lutomirski
2018-07-29 12:00     ` Rik van Riel
     [not found]       ` <E710FBA5-CC5E-4941-ACBF-4AB3424F1F68@amacapital.net>
2018-07-29 17:39         ` Rik van Riel
2018-07-29 17:51         ` Rik van Riel
2018-07-29 18:55           ` Andy Lutomirski
2018-07-29 19:56             ` Linus Torvalds
2018-07-28 21:53 ` [PATCH 04/10] x86,mm: use on_each_cpu_cond for TLB flushes Rik van Riel
2018-07-29  2:58   ` Andy Lutomirski
2018-07-29 12:02     ` Rik van Riel
2018-07-28 21:53 ` [PATCH 05/10] mm,tlb: turn dummy defines into inline functions Rik van Riel
2018-07-28 21:53 ` [PATCH 06/10] mm,x86: skip cr4 and ldt reload when mm stays the same Rik van Riel
2018-07-29  4:21   ` Andy Lutomirski
2018-07-28 21:53 ` [PATCH 07/10] x86,mm: remove leave_mm cpu argument Rik van Riel
2018-07-28 21:53 ` [PATCH 08/10] arch,mm: add config variable to skip lazy TLB mm refcounting Rik van Riel
2018-07-28 21:53 ` [PATCH 09/10] mm,x86: shoot down lazy TLB references at exit_mmap time Rik van Riel
2018-07-28 21:53 ` [PATCH 10/10] mm,sched: conditionally skip lazy TLB mm refcounting Rik van Riel
2018-07-29  4:21   ` Andy Lutomirski [this message]
2018-07-29 12:11     ` Rik van Riel
2018-07-29 15:29       ` Andy Lutomirski
2018-07-29 16:55         ` Rik van Riel
2018-07-29 19:54     ` [PATCH v2 10/11] x86,tlb: really leave mm on shootdown Rik van Riel
2018-07-29 19:54     ` [PATCH v2 11/11] mm,sched: conditionally skip lazy TLB mm refcounting Rik van Riel
2018-07-30  9:55       ` Peter Zijlstra
2018-07-30 14:30         ` Rik van Riel
2018-07-30 16:26           ` Peter Zijlstra
2018-07-30 19:15             ` Rik van Riel
2018-07-30 19:30               ` Andy Lutomirski
2018-07-30 19:36                 ` Rik van Riel
2018-07-30 19:49                   ` Andy Lutomirski
2018-07-30 21:46                     ` Rik van Riel
2018-07-30 22:00                       ` Andy Lutomirski
2018-07-31  1:05             ` Rik van Riel
2018-07-31  9:12               ` Peter Zijlstra
2018-07-31 14:29                 ` Andy Lutomirski
2018-07-31 15:03                   ` Rik van Riel
2018-07-31 15:12                     ` Peter Zijlstra
2018-07-30 11:32 ` [PATCH 0/10] x86,tlb,mm: more lazy TLB cleanups & optimizations Ingo Molnar

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