From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752887AbdFUQUf (ORCPT <rfc822;w@1wt.eu>);
        Wed, 21 Jun 2017 12:20:35 -0400
Received: from mail.kernel.org ([198.145.29.99]:36278 "EHLO mail.kernel.org"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1751133AbdFUPRU (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 21 Jun 2017 11:17:20 -0400
DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0EEFC217C3
Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org
Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=luto@kernel.org
MIME-Version: 1.0
In-Reply-To: <alpine.DEB.2.20.1706211115580.2328@nanos>
References: <cover.1498022414.git.luto@kernel.org> <2b3572123ab0d0fb9a9b82dc0deee8a33eeac51f.1498022414.git.luto@kernel.org>
 <alpine.DEB.2.20.1706211115580.2328@nanos>
From: Andy Lutomirski <luto@kernel.org>
Date: Wed, 21 Jun 2017 08:16:58 -0700
X-Gmail-Original-Message-ID: <CALCETrXmyZ0KwAHbYRwv=hO3nkgctjnb3z5tXvkQKkoABrKgHg@mail.gmail.com>
Message-ID: <CALCETrXmyZ0KwAHbYRwv=hO3nkgctjnb3z5tXvkQKkoABrKgHg@mail.gmail.com>
Subject: Re: [PATCH v3 07/11] x86/mm: Stop calling leave_mm() in idle code
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Nadav Amit <nadav.amit@gmail.com>, Rik van Riel <riel@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Jun 21, 2017 at 2:22 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Tue, 20 Jun 2017, Andy Lutomirski wrote:
>> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
>> index 216d7ec88c0c..2ae43f59091d 100644
>> --- a/drivers/idle/intel_idle.c
>> +++ b/drivers/idle/intel_idle.c
>> @@ -912,16 +912,15 @@ static __cpuidle int intel_idle(struct cpuidle_device *dev,
>>       struct cpuidle_state *state = &drv->states[index];
>>       unsigned long eax = flg2MWAIT(state->flags);
>>       unsigned int cstate;
>> -     int cpu = smp_processor_id();
>>
>>       cstate = (((eax) >> MWAIT_SUBSTATE_SIZE) & MWAIT_CSTATE_MASK) + 1;
>>
>>       /*
>> -      * leave_mm() to avoid costly and often unnecessary wakeups
>> -      * for flushing the user TLB's associated with the active mm.
>> +      * NB: if CPUIDLE_FLAG_TLB_FLUSHED is set, this idle transition
>> +      * will probably flush the TLB.  It's not guaranteed to flush
>> +      * the TLB, though, so it's not clear that we can do anything
>> +      * useful with this knowledge.
>
> So my understanding here is:
>
>       The C-state transition might flush the TLB, when cstate->flags has
>       CPUIDLE_FLAG_TLB_FLUSHED set. The idle transition already took the
>       CPU out of the set of CPUs which are remotely flushed, so the
>       knowledge about this potential flush is not useful for the kernels
>       view of the TLB state.

Indeed.  I assume the theory behind the old code was that leave_mm()
was expensive and that CPUIDLE_FLAG_TLB_FLUSHED would be a decent
heuristic for when to do it.