linux-kernel - Re: [PATCH] rcu/tree: consider time a VM was suspended

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Date:   Fri, 21 May 2021 07:02:27 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Suleiman Souhlal <suleiman@...gle.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu/tree: consider time a VM was suspended

On Fri, May 21, 2021 at 03:42:31PM +0900, Sergey Senozhatsky wrote:
> On (21/05/20 17:14), Paul E. McKenney wrote:
> > > On a side note.
> > > 
> > > I wish we didn't have to put kvm_check_and_clear_guest_paused() all
> > > over the place.
> > > 
> > > We do load jiffies at the start of check_cpu_stall(). So, in theory,
> > > we can just use that captured jiffies (which can become obsolete but
> > > so will grace period timestamps) value and never read current system
> > > jiffies because they can jump way forward. IOW
> > > 
> > > 	jn = j + 3 * rcu_jiffies_till_stall_check() + 3;
> > > 
> > > instead of
> > > 
> > > 	jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
> > > 
> > > Then we probably can remove kvm_check_and_clear_guest_paused().
> > > 
> > > But that "don't load current jiffies" is rather fragile.
> > > 
> > > kvm_check_and_clear_guest_paused() is not pretty, but at least it's
> > > explicit.
> > 
> > If this works for you, I am very much in favor!
> 
> Oh, no. Sorry for the noise. This is racy and won't work.

I had been hoping!  ;-)

							Thanx, Paul