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From: Andy Lutomirski <luto@amacapital.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Nicholas Piggin <npiggin@gmail.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	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: Wed, 2 Dec 2020 06:38:12 -0800	[thread overview]
Message-ID: <BA2FB4C0-55EA-481A-824C-95B94EA29FAB@amacapital.net> (raw)
In-Reply-To: <20201202141957.GJ3021@hirez.programming.kicks-ass.net>


> On Dec 2, 2020, at 6:20 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> On Sun, Nov 29, 2020 at 02:01:39AM +1000, Nicholas Piggin wrote:
>> +         * - A delayed freeing and RCU-like quiescing sequence based on
>> +         *   mm switching to avoid IPIs completely.
> 
> That one's interesting too. so basically you want to count switch_mm()
> invocations on each CPU. Then, periodically snapshot the counter on each
> CPU, and when they've all changed, increment a global counter.
> 
> Then, you snapshot the global counter and wait for it to increment
> (twice I think, the first increment might already be in progress).
> 
> The only question here is what should drive this machinery.. the tick
> probably.
> 
> This shouldn't be too hard to do I think.
> 
> Something a little like so perhaps?

I don’t think this will work.  A CPU can go idle with lazy mm and nohz forever.  This could lead to unbounded memory use on a lightly loaded system.


WARNING: multiple messages have this Message-ID (diff)
From: Andy Lutomirski <luto@amacapital.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-arch@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	Nicholas Piggin <npiggin@gmail.com>,
	linux-mm@kvack.org,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH 6/8] lazy tlb: shoot lazies, a non-refcounting lazy tlb option
Date: Wed, 2 Dec 2020 06:38:12 -0800	[thread overview]
Message-ID: <BA2FB4C0-55EA-481A-824C-95B94EA29FAB@amacapital.net> (raw)
In-Reply-To: <20201202141957.GJ3021@hirez.programming.kicks-ass.net>


> On Dec 2, 2020, at 6:20 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> On Sun, Nov 29, 2020 at 02:01:39AM +1000, Nicholas Piggin wrote:
>> +         * - A delayed freeing and RCU-like quiescing sequence based on
>> +         *   mm switching to avoid IPIs completely.
> 
> That one's interesting too. so basically you want to count switch_mm()
> invocations on each CPU. Then, periodically snapshot the counter on each
> CPU, and when they've all changed, increment a global counter.
> 
> Then, you snapshot the global counter and wait for it to increment
> (twice I think, the first increment might already be in progress).
> 
> The only question here is what should drive this machinery.. the tick
> probably.
> 
> This shouldn't be too hard to do I think.
> 
> Something a little like so perhaps?

I don’t think this will work.  A CPU can go idle with lazy mm and nohz forever.  This could lead to unbounded memory use on a lightly loaded system.


  reply	other threads:[~2020-12-02 14:39 UTC|newest]

Thread overview: 104+ 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 ` Nicholas Piggin
2020-11-28 16:01 ` [PATCH 1/8] lazy tlb: introduce exit_lazy_tlb Nicholas Piggin
2020-11-28 16:01   ` Nicholas Piggin
2020-11-29  0:38   ` Andy Lutomirski
2020-11-29  0:38     ` Andy Lutomirski
2020-11-29  0:38     ` Andy Lutomirski
2020-12-02  2:49     ` Nicholas Piggin
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 16:01   ` Nicholas Piggin
2020-11-28 17:55   ` Andy Lutomirski
2020-11-28 17:55     ` Andy Lutomirski
2020-11-28 17:55     ` Andy Lutomirski
2020-12-02  2:49     ` Nicholas Piggin
2020-12-02  2:49       ` Nicholas Piggin
2020-12-03  5:09       ` Andy Lutomirski
2020-12-03  5:09         ` Andy Lutomirski
2020-12-03  5:09         ` Andy Lutomirski
2020-12-05  8:00         ` Nicholas Piggin
2020-12-05  8:00           ` Nicholas Piggin
2020-12-05 16:11           ` Andy Lutomirski
2020-12-05 16:11             ` Andy Lutomirski
2020-12-05 23:14             ` Nicholas Piggin
2020-12-05 23:14               ` Nicholas Piggin
2020-12-06  0:36               ` Andy Lutomirski
2020-12-06  0:36                 ` Andy Lutomirski
2020-12-06  0:36                 ` Andy Lutomirski
2020-12-06  3:59                 ` Nicholas Piggin
2020-12-06  3:59                   ` Nicholas Piggin
2020-12-11  0:11                   ` Andy Lutomirski
2020-12-11  0:11                     ` Andy Lutomirski
2020-12-11  0:11                     ` Andy Lutomirski
2020-12-14  4:07                     ` Nicholas Piggin
2020-12-14  4:07                       ` Nicholas Piggin
2020-12-14  5:53                       ` Nicholas Piggin
2020-12-14  5:53                         ` Nicholas Piggin
2020-11-30 14:57   ` Mathieu Desnoyers
2020-11-30 14:57     ` Mathieu Desnoyers
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   ` 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   ` Nicholas Piggin
2020-11-28 16:01 ` [PATCH 5/8] lazy tlb: allow lazy tlb mm switching to be configurable Nicholas Piggin
2020-11-28 16:01   ` Nicholas Piggin
2020-11-29  0:36   ` Andy Lutomirski
2020-11-29  0:36     ` Andy Lutomirski
2020-11-29  0:36     ` Andy Lutomirski
2020-12-02  2:49     ` Nicholas Piggin
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
2020-11-28 16:01   ` Nicholas Piggin
2020-11-29  3:54   ` Andy Lutomirski
2020-11-29  3:54     ` Andy Lutomirski
2020-11-29  3:54     ` Andy Lutomirski
2020-11-29 20:16     ` Andy Lutomirski
2020-11-29 20:16       ` Andy Lutomirski
2020-11-29 20:16       ` Andy Lutomirski
2020-11-30  9:25       ` Peter Zijlstra
2020-11-30  9:25         ` Peter Zijlstra
2020-11-30 18:31       ` Andy Lutomirski
2020-11-30 18:31         ` Andy Lutomirski
2020-11-30 18:31         ` Andy Lutomirski
2020-12-01 21:27         ` Will Deacon
2020-12-01 21:27           ` Will Deacon
2020-12-01 21:50           ` Andy Lutomirski
2020-12-01 21:50             ` Andy Lutomirski
2020-12-01 21:50             ` Andy Lutomirski
2020-12-01 23:04             ` Will Deacon
2020-12-01 23:04               ` Will Deacon
2020-12-02  3:47         ` Nicholas Piggin
2020-12-02  3:47           ` Nicholas Piggin
2020-12-03  5:05           ` Andy Lutomirski
2020-12-03  5:05             ` Andy Lutomirski
2020-12-03  5:05             ` Andy Lutomirski
2020-12-03 17:03         ` Alexander Gordeev
2020-12-03 17:03           ` Alexander Gordeev
2020-12-03 17:14           ` Andy Lutomirski
2020-12-03 17:14             ` Andy Lutomirski
2020-12-03 18:33             ` Alexander Gordeev
2020-12-03 18:33               ` Alexander Gordeev
2020-11-30  9:26     ` Peter Zijlstra
2020-11-30  9:26       ` Peter Zijlstra
2020-11-30  9:30     ` Peter Zijlstra
2020-11-30  9:30       ` Peter Zijlstra
2020-11-30  9:34       ` Peter Zijlstra
2020-11-30  9:34         ` Peter Zijlstra
2020-12-02  3:09     ` Nicholas Piggin
2020-12-02  3:09       ` Nicholas Piggin
2020-12-02 11:17   ` Peter Zijlstra
2020-12-02 11:17     ` Peter Zijlstra
2020-12-02 12:45     ` Peter Zijlstra
2020-12-02 12:45       ` Peter Zijlstra
2020-12-02 14:19   ` Peter Zijlstra
2020-12-02 14:19     ` Peter Zijlstra
2020-12-02 14:38     ` Andy Lutomirski [this message]
2020-12-02 14:38       ` Andy Lutomirski
2020-12-02 16:29       ` Peter Zijlstra
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   ` Nicholas Piggin
2020-11-28 16:01 ` [PATCH 8/8] powerpc/64s: enable MMU_LAZY_TLB_SHOOTDOWN Nicholas Piggin
2020-11-28 16:01   ` Nicholas Piggin

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