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Subject: Re: Question about qspinlock nest
To:     Peter Zijlstra <peterz@infradead.org>,
        James Morse <james.morse@arm.com>
Cc:     Zhenzhong Duan <zhenzhong.duan@oracle.com>,
        LKML <linux-kernel@vger.kernel.org>,
        SRINIVAS <srinivas.eeda@oracle.com>,
        Borislav Petkov <bp@alien8.de>,
        Steven Rostedt <rostedt@goodmis.org>
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From:   Waiman Long <longman@redhat.com>
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Message-ID: <abf2dfe7-e148-b011-764d-b9effa573d5d@redhat.com>
Date:   Fri, 18 Jan 2019 09:50:12 -0500
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On 01/18/2019 05:02 AM, Peter Zijlstra wrote:
>
>> e.g. We can't take an SError during the SError handler.
>>
>> But we can take this SError/NMI on another CPU while the first one is still
>> running the handler.
>>
>> These multiple NMIlike notifications mean having multiple locks/fixmap-slots,
>> one per notification. This is where the qspinlock node limit comes in, as we
>> could have more than 4 contexts.
> Right; so Waiman was going to do a patch that reverts to test-and-set or
> something along those lines once we hit the queue limit, which seems
> like a good way out. Actually hitting that nesting level should be
> exceedingly rare.

Yes, I am working on a patch to support arbitrary levels of nesting. It
is easy for PV qspinlock as lock stealing is supported.

For native qspinlock, we cannot do lock stealing without incurring a
certain amount of overhead in the regular slowpath code. It was up to
10% in my own testing. So I am exploring an alternative that can do the
job without incurring any noticeable performance degradation in the
slowpath. I ran into a race condition which I am still trying to find
out where that comes from. Hopefully, I will have something to post next
week.

Cheers,
Longman