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

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Message-ID: <315b0b80f7f541b99a49a9fecb743874f31d95ba.camel@kernel.org>
Date:   Mon, 30 May 2022 20:02:57 +0200
From:   nicolas saenz julienne <nsaenz@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Phil Auld <pauld@...hat.com>,
        Alex Belits <abelits@...vell.com>,
        Xiongfeng Wang <wangxiongfeng2@...wei.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yu Liao <liaoyu15@...wei.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Uladzislau Rezki <uladzislau.rezki@...y.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 20/21] rcu/context_tracking: Merge dynticks counter and
 context tracking states

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@...nel.org>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> Cc: Uladzislau Rezki <uladzislau.rezki@...y.com>
> Cc: Joel Fernandes <joel@...lfernandes.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Nicolas Saenz Julienne <nsaenz@...nel.org>
> Cc: Marcelo Tosatti <mtosatti@...hat.com>
> Cc: Xiongfeng Wang <wangxiongfeng2@...wei.com>
> Cc: Yu Liao<liaoyu15@...wei.com>
> Cc: Phil Auld <pauld@...hat.com>
> Cc: Paul Gortmaker<paul.gortmaker@...driver.com>
> Cc: Alex Belits <abelits@...vell.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.

Regards,
Nicolas