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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 21:55:22 -0500
Message-Id: <D445A93C-8315-4407-A8E4-C377BF63058A@joelfernandes.org>
References: <0BF2065B-1E02-498C-B999-EB52F005B62E@joelfernandes.org>
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: <0BF2065B-1E02-498C-B999-EB52F005B62E@joelfernandes.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Precedence: bulk


> On Jan 7, 2023, at 9:48 PM, Joel Fernandes <joel@joelfernandes.org> wrote:=

>=20
> =EF=BB=BF
>>> On Jan 7, 2023, at 5:11 PM, Frederic Weisbecker <frederic@kernel.org> wr=
ote:
>>>=20
>>> =EF=BB=BFOn Fri, Jan 06, 2023 at 07:01:28PM -0500, Joel Fernandes wrote:=

>>> (lost html content)
>=20
> My problem is the iPhone wises up when I put a web link in an email. I wan=
t to look into smtp relays but then if I spent time on fixing that, I might n=
ot get time to learn from emails like these...=20
>=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.
>=20
> Ok.
>=20
>> 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_PRE=
EMPT=3Dn
>> (and there is also PREEMPT_DYNAMIC to consider).
>=20
> Makes sense. I think I was missing this use case and was going by the gene=
ral design of exp grace periods.  I was incorrectly assuming the IPIs were b=
eing sent repeatedly for hold out CPUs, which is not the case I think. But t=
hat would another way to fix it?
>=20
> But yeah I get your point, the first set of IPIs missed it, so we need the=
 rescue-tick for long non-rcu_read_lock() implicit critical sections..=20
>=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().
>=20
> 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.

I think meant I here, an atomic section (like bh or Irq disabled). There is n=
o such thing as disabling preemption for CONFIG_PREEMPT=3Dn. Or maybe I am c=
onfused again.  This RCU thing=E2=80=A6

Thanks.


>=20
> We ought to start another Google doc on all of this if we have not yet=E2=80=
=A6
>=20
> Thanks!
>=20
> - Joel
>=20
>>=20
>> Thanks.
>>=20
>>=20