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

From: nicolas saenz julienne <nsaenz@kernel.org>
To: Frederic Weisbecker <frederic@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Phil Auld <pauld@redhat.com>, Alex Belits <abelits@marvell.com>,
	Xiongfeng Wang <wangxiongfeng2@huawei.com>,
	Neeraj Upadhyay <quic_neeraju@quicinc.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Yu Liao <liaoyu15@huawei.com>, Boqun Feng <boqun.feng@gmail.com>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Paul Gortmaker <paul.gortmaker@windriver.com>,
	Uladzislau Rezki <uladzislau.rezki@sony.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH 20/21] rcu/context_tracking: Merge dynticks counter and context tracking states
Date: Mon, 30 May 2022 20:02:57 +0200	[thread overview]
Message-ID: <315b0b80f7f541b99a49a9fecb743874f31d95ba.camel@kernel.org> (raw)
In-Reply-To: <20220519145823.618983-21-frederic@kernel.org>

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.

Regards,
Nicolas