linux-kernel - Re: [PATCH 10/21] context_tracking: Take idle eqs entrypoints over RCU

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Date:   Tue, 3 May 2022 14:33:22 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Phil Auld <pauld@...hat.com>,
        Alex Belits <abelits@...vell.com>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        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>
Subject: Re: [PATCH 10/21] context_tracking: Take idle eqs entrypoints over
 RCU

On Tue, May 03, 2022 at 01:42:25PM +0200, Frederic Weisbecker wrote:
> On Tue, May 03, 2022 at 12:02:51PM +0100, Mark Rutland wrote:
> > Hi Frederic,
> > 
> > On Tue, May 03, 2022 at 12:00:40PM +0200, Frederic Weisbecker wrote:
> > > The RCU dynticks counter is going to be merged into the context tracking
> > > subsystem. Start with moving the idle extended quiescent states
> > > entrypoints to context tracking. For now those are dumb redirections to
> > > existing RCU calls.
> > 
> > I was a bit confused looking at this, because that redirection only exists for
> > CONFIG_CONTEXT_TRACKING, and is empty otherwise.
> > 
> > I see this patch makes TREE_RCU select CONTEXT_TRACKING, which means that
> > works. Since that also means building the rest of the context tracking code, I
> > think it'd be worth mentioning that in the commit message.
> 
> Not exactly, it only builds a minimal version of CONTEXT_TRACKING, which maps
> to roughly the vanilla RCU-idle code.
> 
> The user tracking part (which is the vanilla CONTEXT_TRACKING) now becomes
> CONFIG_CONTEXT_TRACKING_USER

Ah, sorry. That makes sense to me now!

> > Do all architectures which can use TREE_RCU today already support context
> > tracking? If not, do those work by default?
> 
> No need to, the old context tracking becomes CONFIG_CONTEXT_TRACKING_USER
> which is not needed by TREE_RCU.
> 
> Ok you're right, this is all very confusing :o)
> 
> Let's clarify this, I'm going to do the following on the next version:
> 
> * The current CONFIG_CONTEXT_TRACKING becomes CONFIG_CONTEXT_TRACKING_USER.
>   This part needs support from the architecture.
> 
> * The RCU-idle functions that have been moved to context tracking will become
>   CONFIG_CONTEXT_TRACKING_IDLE. This part doesn't need support from the
>   architecture and will be set by TREE_RCU.
> 
> Would that look clearer?

I think so, yes!

It would also help if the prior commit message could be explicit about
the end result: After the Kconfig rework both
CONFIG_CONTEXT_TRACKING_USER and CONFIG_CONTEXT_TRACKING exist, but
CONFIG_CONTEXT_TRACKING will only used to decide whether to build object
files with common context tracking logic.

Thanks,
Mark.