| 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
| ||
|
Date: Mon, 27 Sep 2010 14:11:25 -0700
From: Venkatesh Pallipadi <venki@...gle.com>
To: Eric Dumazet <eric.dumazet@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Martin Schwidefsky <schwidefsky@...ibm.com>,
Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Balbir Singh <balbir@...ux.vnet.ibm.com>,
linux-kernel@...r.kernel.org, Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH] si time accounting accounts bh_disable'd time to si
On Mon, Sep 27, 2010 at 1:53 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le lundi 27 septembre 2010 à 13:35 -0700, Venkatesh Pallipadi a écrit :
>> >> >> > You still do have the problem with local_bh_disable() though, since you
>> >> >> > cannot distinguish between having bh disabled and processing softirq.
>> >> >> >
>> >> >> > So a hardirq that hits while you have bh disabled will inflate your
>> >> >> > softirq time.
>>
>> >> >> Hmm, that bug is valid for CONFIG_VIRT_CPU_ACCOUNTING=y as well.
>> >> >
>> >> > And nobody ever noticed?
>> >> >
>> >> Yes. I inherited the API from VIRT_CPU_ACCOUNTING along with this
>> >> local_bh_disable bug. Agree that we need one extra bit to handle this
>> >> case. I will take a stab at fixing this along with refresh of this
>> >> patchset if no one else has beaten me to it until then.
>> >
>> >Make sure to only fix the softirq processing on the hardirq tail, not
>> > the ksoftirqd one :-)
>>
>> softirq processing from hardirq tail and ksoftirqd are currently
>> handled in the same way and I didn't see any issues changing both of
>> them. Am I missing something?
>>
>> Here's the patch I have for this.
>>
>> [PATCH] si time accounting accounts bh_disable'd time to si
>>
>> Peter Zijlstra found a bug in the way softirq time is accounted in
>> VIRT_CPU_ACCOUNTING on this thread.
>> http://lkml.indiana.edu/hypermail//linux/kernel/1009.2/01366.html
>>
>> The problem is, softirq processing uses local_bh_disable internally and there
>> would be no way later in the flow to differentiate between whether softirq is
>> being processed or is it just that bh has been disabled. So, a hardirq when bh
>> id disabled results in time being wrongly accounted for softirq.
>>
>> Looking at the code a bit more, the problem exists in !VIRT_CPU_ACCOUNTING
>> as well. As account_system_time() in normal tick based accouting also uses
>> softirq_count, which will be set when not in softirq with bh disabled.
>>
>> Peter also suggested solution of using 2 * SOFTIRQ_OFFSET as irq count
>> for local_bh_{disable,enable} and using just SOFTIRQ_OFFSET while softirq
>> processing. The patch below does that and adds API in_serving_softirq() which
>> returns whether we are currently processing softirq or not.
>>
>> Also changes one of the usages of softirq_count in net/sched/cls_cgroup.c
>> to in_serving_softirq.
>>
>> Looks like many usages of in_softirq really want in_serving_softirq. Those
>> changes can be made individually on a case by case basis.
>>
>> Signed-off-by: Venkatesh Pallipadi <venki@...gle.com>
>> ---
>> include/linux/hardirq.h | 3 ++
>> include/linux/sched.h | 2 +-
>> kernel/sched.c | 2 +-
>> kernel/softirq.c | 51 +++++++++++++++++++++++++++++++---------------
>> net/sched/cls_cgroup.c | 2 +-
>> 5 files changed, 40 insertions(+), 20 deletions(-)
>>
>> diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
>> index d5b3876..1c736ae 100644
>> --- a/include/linux/hardirq.h
>> +++ b/include/linux/hardirq.h
>> @@ -82,10 +82,13 @@
>> /*
>> * Are we doing bottom half or hardware interrupt processing?
>> * Are we in a softirq context? Interrupt context?
>> + * in_softirq answers - are we currently processing softirq or have bh disabled?
>> + * in_serving_softirq answers - are we currently processing softirq?
>> */
>> #define in_irq() (hardirq_count())
>> #define in_softirq() (softirq_count())
>> #define in_interrupt() (irq_count())
>> +#define in_serving_softirq() (softirq_count() == SOFTIRQ_OFFSET)
>>
>
> But softirq handlers sometime call functions that might disable bh
> again. It would be good to not switch softirq time to system time ;)
Yes. Good point :-). I should rather have
+#define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET)
>
> Shouldnt we reserve a bit (high order bit out of 8) instead ?
>
> * PREEMPT_MASK: 0x000000ff
> * SOFTIRQ_MASK: 0x0000ff00
> * SERVING_SOFTIRQ: 0x00008000
> * HARDIRQ_MASK: 0x03ff0000
> * NMI_MASK: 0x04000000
Things will be very much similar using higher order bit or lower order
bit. Somehow I felt using lower order bit was cleaner...
Thanks,
Venki
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists