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Subject: Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance
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From:   Waiman Long <longman@redhat.com>
To:     Alex Kogan <alex.kogan@oracle.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, linux@armlinux.org.uk,
        Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Arnd Bergmann <arnd@arndb.de>, linux-arch@vger.kernel.org,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, hpa@zytor.com, x86@kernel.org,
        Hanjun Guo <guohanjun@huawei.com>,
        Jan Glauber <jglauber@marvell.com>,
        Steven Sistare <steven.sistare@oracle.com>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        dave.dice@oracle.com
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Date:   Sat, 25 Jan 2020 14:57:41 -0500
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On 1/24/20 1:51 PM, Waiman Long wrote:
>> You can use the in_task() macro in include/linux/preempt.h. This is
>> just a percpu preempt_count read and test. If in_task() is false, it
>> is in a {soft|hard}irq or nmi context. If it is true, you can check
>> the rt_task() macro to see if it is an RT task. That will access to
>> the current task structure. So it may cost a little bit more if you
>> want to handle the RT task the same way.
>>
> We may not need to do that for softIRQ context. If that is the case, yo=
u
> can use in_irq() which checks for hardirq and nmi only. Peter, what is
> your thought on that?

In second thought, we should do that for softIRQ as well. Also, we may
want to also check if irqs_disabled() is true as well by calls like
spin_lock_irq() or spin_lock_irqsave().=C2=A0 We do not want to unnecessa=
rily
prolong the irq off period.

Cheers,
Longman