Re: [PATCH 20/21] rcu/context_tracking: Merge dynticks counter and context tracking states

Frederic Weisbecker
2022-05-31T14:23:48Z

On Mon, May 30, 2022 at 08:02:57PM +0200, nicolas saenz julienne wrote:
> Hi Frederic,
>
> On Thu, 2022-05-19 at 16:58 +0200, Frederic Weisbecker wrote:
> > Updating the context tracking state and the RCU dynticks counter
> > atomically in a single operation is a first step towards improving CPU
> > isolation. This makes the context tracking state updates fully ordered
> > and therefore allow for later enhancements such as postponing some work
> > while a task is running isolated in userspace until it ever comes back
> > to the kernel.
> >
> > The state field becomes divided in two parts:
> >
> > 1) Two Lower bits for context tracking state:
> >
> > CONTEXT_KERNEL = 0
> > CONTEXT_IDLE = 1,
> > CONTEXT_USER = 2,
> > CONTEXT_GUEST = 3,
> >
> > 2) Higher bits for RCU eqs dynticks counting:
> >
> > RCU_DYNTICKS_IDX = 4
> >
> > The dynticks counting is always incremented by this value.
> > (state & RCU_DYNTICKS_IDX) means we are NOT in an extended quiescent
> > state. This makes the chance for a collision more likely between two
> > RCU dynticks snapshots but wrapping up 28 bits of eqs dynticks
> > increments still takes some bad luck (also rdp.dynticks_snap could be
> > converted from int to long?)
> >
> > Some RCU eqs functions have been renamed to better reflect their broader
> > scope that now include context tracking state.
> >
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> > Cc: Uladzislau Rezki <uladzislau.rezki@sony.com>
> > Cc: Joel Fernandes <joel@joelfernandes.org>
> > Cc: Boqun Feng <boqun.feng@gmail.com>
> > Cc: Nicolas Saenz Julienne <nsaenz@kernel.org>
> > Cc: Marcelo Tosatti <mtosatti@redhat.com>
> > Cc: Xiongfeng Wang <wangxiongfeng2@huawei.com>
> > Cc: Yu Liao<liaoyu15@huawei.com>
> > Cc: Phil Auld <pauld@redhat.com>
> > Cc: Paul Gortmaker<paul.gortmaker@windriver.com>
> > Cc: Alex Belits <abelits@marvell.com>
> > ---
>
> While working on a feature on top of this series (IPI deferral stuff) I believe
> I've found a discrepancy on how context state is being updated:
>
> - When servicing an IRQ from user-space, we increment dynticks, and clear the
> ct state to show we're in-kernel.
>
> - When servicing an IRQ from idle/guest or an NMI from any context we only
> increment the dynticks counter. The ct state remains unchanged.

Hmm, an IRQ from userspace does:

ct_user_enter()
//run in user
//-----IRQ
ct_user_exit()
ct_irq_enter()
ct_irq_exit()
ct_user_enter()
//run in user

An IRQ from guest does:

for (;;) {
context_tracking_guest_enter()
//vmrun
//IRQ pending
#VMEXIT
context_tracking_guest_exit()
local_irq_enable()
ct_irq_enter()
ct_irq_exit()
local_irq_disable()
}

(although I see there is an "sti" right before "vmrun" so it looks
possible to have ct_irq_enter() after context_tracking_guest_enter()
if a host IRQ fires between the sti and the vmrun though I might be
missing some kvm subtelty).

An IRQ from idle does just:

ct_idle_enter()
//IRQ
ct_irq_enter()
ct_irq_exit()
ct_idle_exit()

So guest looks mostly ok to me (except for the little sti before vmrun for
which I have a doubt). But idle at least is an exception and CONTEXT_IDLE will
remain during the interrupt handling. It's not that trivial to handle the idle
case because ct_irq_exit() needs to know that it is called between
ct_idle_enter() and ct_idle_exit().

Thanks.