Re: [RFC PATCH v2 16/20] rcu: Make RCU dynticks counter size configurable

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Valentin Schneider <vschneid@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
	linux-doc@vger.kernel.org, kvm@vger.kernel.org,
	linux-mm@kvack.org, bpf@vger.kernel.org, x86@kernel.org,
	rcu@vger.kernel.org, linux-kselftest@vger.kernel.org,
	"Steven Rostedt" <rostedt@goodmis.org>,
	"Masami Hiramatsu" <mhiramat@kernel.org>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Wanpeng Li" <wanpengli@tencent.com>,
	"Vitaly Kuznetsov" <vkuznets@redhat.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Frederic Weisbecker" <frederic@kernel.org>,
	"Neeraj Upadhyay" <quic_neeraju@quicinc.com>,
	"Joel Fernandes" <joel@joelfernandes.org>,
	"Josh Triplett" <josh@joshtriplett.org>,
	"Boqun Feng" <boqun.feng@gmail.com>,
	"Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>,
	"Lai Jiangshan" <jiangshanlai@gmail.com>,
	Zqiang <qiang.zhang1211@gmail.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Uladzislau Rezki" <urezki@gmail.com>,
	"Christoph Hellwig" <hch@infradead.org>,
	"Lorenzo Stoakes" <lstoakes@gmail.com>,
	"Josh Poimboeuf" <jpoimboe@kernel.org>,
	"Jason Baron" <jbaron@akamai.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Sami Tolvanen" <samitolvanen@google.com>,
	"Ard Biesheuvel" <ardb@kernel.org>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Juerg Haefliger" <juerg.haefliger@canonical.com>,
	"Nicolas Saenz Julienne" <nsaenz@kernel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	"Nadav Amit" <namit@vmware.com>,
	"Dan Carpenter" <error27@gmail.com>,
	"Chuang Wang" <nashuiliang@gmail.com>,
	"Yang Jihong" <yangjihong1@huawei.com>,
	"Petr Mladek" <pmladek@suse.com>,
	"Jason A. Donenfeld" <Jason@zx2c4.com>,
	"Song Liu" <song@kernel.org>,
	"Julian Pidancet" <julian.pidancet@oracle.com>,
	"Tom Lendacky" <thomas.lendacky@amd.com>,
	"Dionna Glaze" <dionnaglaze@google.com>,
	"Thomas Weißschuh" <linux@weissschuh.net>,
	"Juri Lelli" <juri.lelli@redhat.com>,
	"Daniel Bristot de Oliveira" <bristot@redhat.com>,
	"Marcelo Tosatti" <mtosatti@redhat.com>,
	"Yair Podemsky" <ypodemsk@redhat.com>
Subject: Re: [RFC PATCH v2 16/20] rcu: Make RCU dynticks counter size configurable
Date: Fri, 21 Jul 2023 09:09:13 -0700	[thread overview]
Message-ID: <ac2ec330-191e-43e6-817d-ca4a22207d98@paulmck-laptop> (raw)
In-Reply-To: <xhsmhbkg5ti91.mognet@vschneid.remote.csb>

On Fri, Jul 21, 2023 at 04:08:10PM +0100, Valentin Schneider wrote:
> On 21/07/23 07:10, Paul E. McKenney wrote:
> > On Fri, Jul 21, 2023 at 09:17:53AM +0100, Valentin Schneider wrote:
> >> On 20/07/23 17:30, Valentin Schneider wrote:
> >> > index bdd7eadb33d8f..1ff2aab24e964 100644
> >> > --- a/kernel/rcu/Kconfig
> >> > +++ b/kernel/rcu/Kconfig
> >> > @@ -332,4 +332,37 @@ config RCU_DOUBLE_CHECK_CB_TIME
> >> >         Say Y here if you need tighter callback-limit enforcement.
> >> >         Say N here if you are unsure.
> >> >
> >> > +config RCU_DYNTICKS_RANGE_BEGIN
> >> > +	int
> >> > +	depends on !RCU_EXPERT
> >> > +	default 31 if !CONTEXT_TRACKING_WORK
> >>
> >> You'll note that this should be 30 really, because the lower *2* bits are
> >> taken by the context state (CONTEXT_GUEST has a value of 3).
> >>
> >> This highlights the fragile part of this: the Kconfig values are hardcoded,
> >> but they depend on CT_STATE_SIZE, CONTEXT_MASK and CONTEXT_WORK_MAX. The
> >> static_assert() will at least capture any misconfiguration, but having that
> >> enforced by the actual Kconfig ranges would be less awkward.
> >>
> >> Do we currently have a way of e.g. making a Kconfig file depend on and use
> >> values generated by a C header?
> >
> > Why not just have something like a boolean RCU_DYNTICKS_TORTURE Kconfig
> > option and let the C code work out what the number of bits should be?
> >
> > I suppose that there might be a failure whose frequency depended on
> > the number of bits, which might be an argument for keeping something
> > like RCU_DYNTICKS_RANGE_BEGIN for fault isolation.  But still using
> > RCU_DYNTICKS_TORTURE for normal testing.
> >
> > Thoughts?
> >
> 
> AFAICT if we run tests with the minimum possible width, then intermediate
> values shouldn't have much value.
> 
> Your RCU_DYNTICKS_TORTURE suggestion sounds like a saner option than what I
> came up with, as we can let the context tracking code figure out the widths
> itself and not expose any of that to Kconfig.

Agreed.  If a need for variable numbers of bits ever does arise, we can
worry about it at that time.  And then we would have more information
on what a variable-bit facility should look like.

							Thanx, Paul