linux-kernel - Re: [PATCH 4/4] irqtime: drop local_irq_save/restore from irqtime_account

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4738023a-adcf-69b5-5856-12c4dc619363@redhat.com>
Date:	Fri, 8 Jul 2016 16:34:55 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Rik van Riel <riel@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	mingo@...nel.org, fweisbec@...hat.com, wanpeng.li@...mail.com,
	efault@....de, tglx@...utronix.de, rkrcmar@...hat.com
Subject: Re: [PATCH 4/4] irqtime: drop local_irq_save/restore from
 irqtime_account_irq



On 08/07/2016 15:19, Rik van Riel wrote:
> On Fri, 2016-07-08 at 14:30 +0200, Frederic Weisbecker wrote:
>> On Thu, Jun 30, 2016 at 03:35:50PM -0400, riel@...hat.com wrote:
>>> From: Rik van Riel <riel@...hat.com>
>>>
>>> Drop local_irq_save/restore from irqtime_account_irq.
>>> Instead, have softirq and hardirq track their time spent
>>> independently, with the softirq code subtracting hardirq
>>> time that happened during the duration of the softirq run.
>>>
>>> The softirq code can be interrupted by hardirq code at
>>> any point in time, but it can check whether it got a
>>> consistent snapshot of the timekeeping variables it wants,
>>> and loop around in the unlikely case that it did not.
>>>
>>> Signed-off-by: Rik van Riel <riel@...hat.com>
>>
>> So the purpose is to get rid of local_irq_save/restore()?
>> Is it really worth such complication?
> 
> local_irq_save/restore are quite slow, and look like the
> largest source of overhead in irq time accounting.

I'm looking at an upstream tree, without your patches applied,
but it seems to me that irqtime_account_irq is always called with
interrupts disabled:

irqtime_account_irq
	-> account_irq_enter_time
		-> __irq_enter
			-> HARDIRQ_ENTER	[1]
			-> irq_enter		[3]
		-> __do_softirq			[1]
	-> account_irq_exit_time
		-> __do_softirq			[1]
		-> __irq_exit
			-> HARDIRQ_EXIT		[1]
		-> irq_exit			[2]

	[1] = does local_irq_disable/enable
	[2] = contains WARN_ON_ONCE(!irqs_disabled())
	[3] = calls rcu_irq_enter(), which checks irqs_disabled()

I don't think your first two patches change this, so perhaps it's enough
to remove that local_irq-save/restore?  Either this, or ENEEDWEEKEND...

Paolo



Download attachment "signature.asc" of type "application/pgp-signature" (474 bytes)