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Subject: Re: Gang scheduling
To:     Tim Chen <tim.c.chen@linux.intel.com>, linux-kernel@vger.kernel.org
Cc:     Peter Zijlstra <peterz@infradead.org>
References: <44347e72-d595-5c2d-3983-7237526c71c6@oracle.com>
 <c08136ad-3e68-ce1c-af03-b591acc582fc@linux.intel.com>
From:   Subhra Mazumdar <subhra.mazumdar@oracle.com>
Message-ID: <890bd498-93b2-5d9d-8066-21278540727f@oracle.com>
Date:   Tue, 12 Feb 2019 18:57:49 -0800
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Hi Tim,

On 10/12/18 11:01 AM, Tim Chen wrote:
> On 10/10/2018 05:09 PM, Subhra Mazumdar wrote:
>> Hi,
>>
>> I was following the Coscheduling patch discussion on lkml and Peter mentioned he had a patch series. I found the following on github.
>>
>> https://github.com/pdxChen/gang/commits/sched_1.23-loadbal
>>
>> I would like to test this with KVMs. Are the commits from 38d5acb to f019876 sufficient? Also is there any documentaion on how to use it (any knobs I need to turn on for gang scheduling to happen?) or is it enabled by default for KVMs?
>>
>> Thanks,
>> Subhra
>>
> I would suggest you try
> https://github.com/pdxChen/gang/tree/sched_1.23-base
> without the load balancing part of gang scheduling.
> It is enabled by default for KVMs.
I applied the following 3 patches on 4.19 and tried to install a KVM (with
virt-install). But the kernel hangs with following error:

kernel:watchdog: BUG: soft lockup - CPU#21 stuck for 23s! [kworker/21:1:573]

kvm,sched: Track VCPU threads
x86/kvm,sched: Add fast path for reschedule interrupt
sched: Optimize scheduler_ipi()

The track VCPU patch seems to be the culprit.

Thanks,
Subhra
>
> Due to the constant change in gang scheduling status of the QEMU thread
> depending on whether vcpu is loaded or unloaded,
> the load balancing part of the code doesn't work very well.
>
> The current version of the code need to be optimized further.  Right now
> the QEMU thread constantly does vcpu load and unload during VM enter and exit.
> We gang schedule only after vcpu load and register the thread to be gang
> scheduled.  When we do vcpu unload, the thread is removed from the set
> to be gang scheduled.  Each time there's a synchronization with the
> sibling thread that's expensive.
>
> However, for QEMU, there's a one to one correspondence between the QEMU
> thread and vcpu.  So we don't have to change the gang scheduling status
> for such thread to avoid the church and sync with the sibling. That should
> be helpful for VM with lots of I/O causing constant VM exits.  We're
> still working on this optimization.  And the load balancing should be
> better after this change.
>
> Tim