linux-kernel - Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking readers from consuming CPU) cause qemu boot slow

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Date:   Sun, 12 Jun 2022 19:47:10 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     paulmck@...nel.org,
        "zhangfei.gao@...mail.com" <zhangfei.gao@...mail.com>
Cc:     Zhangfei Gao <zhangfei.gao@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        rcu@...r.kernel.org, Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Matthew Wilcox <willy@...radead.org>,
        Shameerali Kolothum Thodi 
        <shameerali.kolothum.thodi@...wei.com>, mtosatti@...hat.com,
        sheng.yang@...el.com
Subject: Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking
 readers from consuming CPU) cause qemu boot slow

On 6/12/22 19:29, Paolo Bonzini wrote:
> On 6/12/22 18:40, Paul E. McKenney wrote:
>>> Do these reserved memory regions really need to be allocated separately?
>>> (For example, are they really all non-contiguous?  If not, that is, if
>>> there are a lot of contiguous memory regions, could you sort the IORT
>>> by address and do one ioctl() for each set of contiguous memory 
>>> regions?)
>>>
>>> Are all of these reserved memory regions set up before init is spawned?
>>>
>>> Are all of these reserved memory regions set up while there is only a
>>> single vCPU up and running?
>>>
>>> Is the SRCU grace period really needed in this case?  (I freely confess
>>> to not being all that familiar with KVM.)
>>
>> Oh, and there was a similar many-requests problem with networking many
>> years ago.  This was solved by adding a new syscall/ioctl()/whatever
>> that permitted many requests to be presented to the kernel with a single
>> system call.
>>
>> Could a new ioctl() be introduced that requested a large number
>> of these memory regions in one go so as to make each call to
>> synchronize_rcu_expedited() cover a useful fraction of your 9000+
>> requests?  Adding a few of the KVM guys on CC for their thoughts.

Another question: how much can call_srcu() callbacks pile up these days? 
  I've always been a bit wary of letting userspace do an arbitrary 
number of allocations that can only be freed after a grace period, but 
perhaps there's a way to query SRCU and apply some backpressure?

Paolo