linux-kernel - Re: [PATCH] iommu: Avoid softlockup and rcu stall in fq_flush

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Date:   Thu, 16 Feb 2023 19:02:59 +0800
From:   Peng Zhang <zhangpeng.00@...edance.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     robin.murphy@....com, joro@...tes.org, will@...nel.org,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Li Bin <huawei.libin@...wei.com>,
        Xie XiuQi <xiexiuqi@...wei.com>,
        Yang Yingliang <yangyingliang@...wei.com>
Subject: Re: [PATCH] iommu: Avoid softlockup and rcu stall in
 fq_flush_timeout().



在 2023/2/16 16:49, Hillf Danton 写道:
> On Thu, 16 Feb 2023 15:11:48 +0800 Peng Zhang <zhangpeng.00@...edance.com>
>> There is softlockup under fio pressure test with smmu enabled:
>> watchdog: BUG: soft lockup - CPU#81 stuck for 22s!  [swapper/81:0]
> 
> What is your kernel version?
Rcu stall occurs in kernel version 5.4.
The test where the softlockup happened was not done by me, so I don't 
know the kernel version.
However, it is the same as the code logic of fq_flush_timeout in the 
mainline kernel.

>> This is because the timer callback fq_flush_timeout may run more than
>> 10ms, and timer may be processed continuously in the softirq so trigger
>> softlockup and rcu stall. We can use work to deal with fq_ring_free for
>> each cpu which may take long time, that to avoid triggering softlockup
>> and rcu stall.
>>
>> This patch is modified from the patch[1] of openEuler.
> 
> Because of a timer hog observed on your system with 128 CPUs for instance
> does it make any sense to ask Peter to apply the patch for his 2-CPU box?
What is 2-CPU box?