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From:   Joel Fernandes <joel@joelfernandes.org>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH] rcu: Fix missing TICK_DEP_MASK_RCU_EXP dependency check
Date:   Sat, 7 Jan 2023 21:48:46 -0500
Message-Id: <0BF2065B-1E02-498C-B999-EB52F005B62E@joelfernandes.org>
References: <Y7nuFrJ3gjNzXmCz@lothringen>
Cc:     paulmck@kernel.org, Zqiang <qiang1.zhang@intel.com>,
        quic_neeraju@quicinc.com, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org
In-Reply-To: <Y7nuFrJ3gjNzXmCz@lothringen>
To:     Frederic Weisbecker <frederic@kernel.org>
Precedence: bulk


> On Jan 7, 2023, at 5:11 PM, Frederic Weisbecker <frederic@kernel.org> wrot=
e:
>=20
> =EF=BB=BFOn Fri, Jan 06, 2023 at 07:01:28PM -0500, Joel Fernandes wrote:
>> (lost html content)

My problem is the iPhone wises up when I put a web link in an email. I want t=
o look into smtp relays but then if I spent time on fixing that, I might not=
 get time to learn from emails like these...=20

> I can't find a place where the exp grace period sends an IPI to
> CPUs slow to report a QS. But anyway you really need the tick to poll
> periodically on the CPU to chase a quiescent state.

Ok.

> Now arguably it's probably only useful when CONFIG_PREEMPT_COUNT=3Dy
> and rcu_exp_handler() has interrupted a preempt-disabled or bh-disabled
> section. Although rcu_exp_handler() sets TIF_RESCHED, which is handled
> by preempt_enable() and local_bh_enable() when CONFIG_PREEMPT=3Dy.
> So probably it's only useful when CONFIG_PREEMPT_COUNT=3Dy and CONFIG_PREE=
MPT=3Dn
> (and there is also PREEMPT_DYNAMIC to consider).

Makes sense. I think I was missing this use case and was going by the genera=
l design of exp grace periods.  I was incorrectly assuming the IPIs were bei=
ng sent repeatedly for hold out CPUs, which is not the case I think. But tha=
t would another way to fix it?

But yeah I get your point, the first set of IPIs missed it, so we need the r=
escue-tick for long non-rcu_read_lock() implicit critical sections..=20

> If CONFIG_PREEMPT_COUNT=3Dn, the tick can only report idle and user
> as QS, but those are already reported explicitly on ct_kernel_exit() ->
> rcu_preempt_deferred_qs().

Oh hmm, because that function is a NOOP for PREEMPT_COUNT=3Dy and PREEMPT=3D=
n and will not report the deferred QS?  Maybe it should then. However I thin=
k the tick is still useful if after the preempt disabled section, will still=
 did not exit the kernel.

We ought to start another Google doc on all of this if we have not yet=E2=80=
=A6

Thanks!

 - Joel

>=20
> Thanks.
>=20
>=20