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

[nicolas saenz julienne]

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@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> Cc: Uladzislau Rezki <uladzislau.rezki@xxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Nicolas Saenz Julienne <nsaenz@xxxxxxxxxx>
> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Cc: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
> Cc: Yu Liao<liaoyu15@xxxxxxxxxx>
> Cc: Phil Auld <pauld@xxxxxxxxxx>
> Cc: Paul Gortmaker<paul.gortmaker@xxxxxxxxxxxxx>
> Cc: Alex Belits <abelits@xxxxxxxxxxx>
> ---

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.

Regards,
Nicolas