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

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Message-ID: <20220612185154.GV1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Sun, 12 Jun 2022 11:51:54 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "zhangfei.gao@...mail.com" <zhangfei.gao@...mail.com>,
        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 Sun, Jun 12, 2022 at 07:47:10PM +0200, Paolo Bonzini wrote:
> 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?

They can pile up as much as ever, especially if you have long-duration
sleeping readers.

But you could do the occasional srcu_barrier() to wait for all the
preceding ones to get done.  Maybe every 1000th call_srcu() or similar?

							Thanx, Paul