From: Rik van Riel <riel@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>,
Ingo Molnar <mingo@kernel.org>,
Andy Lutomirski <luto@amacapital.net>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
X86 ML <x86@kernel.org>,
williams@redhat.com, Andrew Lutomirski <luto@kernel.org>,
fweisbec@redhat.com, Peter Zijlstra <peterz@infradead.org>,
Heiko Carstens <heiko.carstens@de.ibm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
"Paul E. McKenney" <paulmck@us.ibm.com>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry
Date: Mon, 04 May 2015 09:30:47 -0400 [thread overview]
Message-ID: <55477487.8050209@redhat.com> (raw)
In-Reply-To: <55473B47.6080600@redhat.com>
On 05/04/2015 05:26 AM, Paolo Bonzini wrote:
>
>
> On 02/05/2015 07:27, Ingo Molnar wrote:
>>
>> - synchronize_rcu() avoids having to send an IPI by taking a
>> peak at rq->curr's pt_regs::flag, and if:
>>
>> - the flag is 0 then it has observed a quiescent state.
>>
>> - the flag is 1, then it would set TIF_NOHZ and wait for a
>> completion from a TIF_NOHZ callback.
>
> Isn't this racy?
>
> synchronize_rcu CPU nohz CPU
> ---------------------------------------------------------
> set flag = 0
> read flag = 0
> return to userspace
> set TIF_NOHZ
>
> and there's no guarantee that TIF_NOHZ is ever processed by the nohz CPU.
If the flag and TIF_NOHZ live in the same word, we can cmpxchg
and be guaranteed atomicity.
--
All rights reversed
next prev parent reply other threads:[~2015-05-04 13:31 UTC|newest]
Thread overview: 83+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-30 21:23 [PATCH 0/3] reduce nohz_full syscall overhead by 10% riel
2015-04-30 21:23 ` [PATCH 1/3] reduce indentation in __acct_update_integrals riel
2015-04-30 21:23 ` [PATCH 2/3] remove local_irq_save from __acct_update_integrals riel
2015-04-30 21:23 ` [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry riel
2015-04-30 21:56 ` Andy Lutomirski
2015-05-01 6:40 ` Ingo Molnar
2015-05-01 15:20 ` Rik van Riel
2015-05-01 15:59 ` Ingo Molnar
2015-05-01 16:03 ` Andy Lutomirski
2015-05-01 16:21 ` Ingo Molnar
2015-05-01 16:26 ` Rik van Riel
2015-05-01 16:34 ` Ingo Molnar
2015-05-01 18:05 ` Rik van Riel
2015-05-01 18:40 ` Ingo Molnar
2015-05-01 19:11 ` Rik van Riel
2015-05-01 19:37 ` Andy Lutomirski
2015-05-02 5:27 ` Ingo Molnar
2015-05-02 18:27 ` Rik van Riel
2015-05-03 18:41 ` Andy Lutomirski
2015-05-07 10:35 ` Ingo Molnar
2015-05-04 9:26 ` Paolo Bonzini
2015-05-04 13:30 ` Rik van Riel [this message]
2015-05-04 14:06 ` Rik van Riel
2015-05-04 14:19 ` Rik van Riel
2015-05-04 15:59 ` question about RCU dynticks_nesting Rik van Riel
2015-05-04 18:39 ` Paul E. McKenney
2015-05-04 19:39 ` Rik van Riel
2015-05-04 20:02 ` Paul E. McKenney
2015-05-04 20:13 ` Rik van Riel
2015-05-04 20:38 ` Paul E. McKenney
2015-05-04 20:53 ` Rik van Riel
2015-05-05 5:54 ` Paul E. McKenney
2015-05-06 1:49 ` Mike Galbraith
2015-05-06 3:44 ` Mike Galbraith
2015-05-06 6:06 ` Paul E. McKenney
2015-05-06 6:52 ` Mike Galbraith
2015-05-06 7:01 ` Mike Galbraith
2015-05-07 0:59 ` Frederic Weisbecker
2015-05-07 15:44 ` Rik van Riel
2015-05-04 19:00 ` Rik van Riel
2015-05-04 19:39 ` Paul E. McKenney
2015-05-04 19:59 ` Rik van Riel
2015-05-04 20:40 ` Paul E. McKenney
2015-05-05 10:53 ` Peter Zijlstra
2015-05-05 12:34 ` Paul E. McKenney
2015-05-05 13:00 ` Peter Zijlstra
2015-05-05 18:35 ` Paul E. McKenney
2015-05-05 21:09 ` Rik van Riel
2015-05-06 5:41 ` Paul E. McKenney
2015-05-05 10:48 ` Peter Zijlstra
2015-05-05 10:51 ` Peter Zijlstra
2015-05-05 12:30 ` Paul E. McKenney
2015-05-02 4:06 ` [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry Mike Galbraith
2015-05-01 16:37 ` Ingo Molnar
2015-05-01 16:40 ` Rik van Riel
2015-05-01 16:45 ` Ingo Molnar
2015-05-01 16:54 ` Rik van Riel
2015-05-01 17:12 ` Ingo Molnar
2015-05-01 17:22 ` Rik van Riel
2015-05-01 17:59 ` Ingo Molnar
2015-05-01 16:22 ` Rik van Riel
2015-05-01 16:27 ` Ingo Molnar
2015-05-03 13:23 ` Mike Galbraith
2015-05-03 17:30 ` Rik van Riel
2015-05-03 18:24 ` Andy Lutomirski
2015-05-03 18:52 ` Rik van Riel
2015-05-07 10:48 ` Ingo Molnar
2015-05-07 12:18 ` Frederic Weisbecker
2015-05-07 12:29 ` Ingo Molnar
2015-05-07 15:47 ` Rik van Riel
2015-05-08 7:58 ` Ingo Molnar
2015-05-07 12:22 ` Andy Lutomirski
2015-05-07 12:44 ` Ingo Molnar
2015-05-07 12:49 ` Ingo Molnar
2015-05-08 6:17 ` Paul E. McKenney
2015-05-07 12:52 ` Andy Lutomirski
2015-05-07 15:08 ` Ingo Molnar
2015-05-07 17:47 ` Andy Lutomirski
2015-05-08 6:37 ` Ingo Molnar
2015-05-08 10:59 ` Andy Lutomirski
2015-05-08 11:27 ` Ingo Molnar
2015-05-08 12:56 ` Andy Lutomirski
2015-05-08 13:27 ` Ingo Molnar
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