Received-SPF: pass (google.com: domain of linux-kernel+bounces-169952-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Message-ID: <7d46b465dd986bdf07330a7ed8d466674dec4859.camel@redhat.com>
Subject: Re: [PATCH v2 net-next 00/15] locking: Introduce nested-BH locking.
From: Paolo Abeni <pabeni@redhat.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, "David S. Miller"
 <davem@davemloft.net>, Boqun Feng <boqun.feng@gmail.com>, Daniel Borkmann
 <daniel@iogearbox.net>, Eric Dumazet <edumazet@google.com>, Frederic
 Weisbecker <frederic@kernel.org>, Ingo Molnar <mingo@redhat.com>, Jakub
 Kicinski <kuba@kernel.org>, Peter Zijlstra <peterz@infradead.org>, Thomas
 Gleixner <tglx@linutronix.de>, Waiman Long <longman@redhat.com>, Will
 Deacon <will@kernel.org>
Date: Mon, 06 May 2024 16:12:00 +0200
In-Reply-To: <20240506093828.OLP2KzcG@linutronix.de>
References: <20240503182957.1042122-1-bigeasy@linutronix.de>
	 <0d2eca9e80ae644583248a9e4a4bdee94999a8f8.camel@redhat.com>
	 <20240506093828.OLP2KzcG@linutronix.de>
Autocrypt: addr=pabeni@redhat.com; prefer-encrypt=mutual; keydata=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
 7ZxN9ZsDL38qVIuFkSpgMZCiUHdxBC11J8nbBSLlpnc924UAyr5XrGA99 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
 iWjoCy8L35zPqR5TV+f5WR5hTVCqmLHSgm1jxwKhPg9L+GfuE4d0SWd84y 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
 sD2W1U7eyLdm/jCgticGDsRtweytsgCSYfbz0gdgUuL3EBYN3JLbAU+UZpy 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
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
User-Agent: Evolution 3.50.4 (3.50.4-1.fc39) 
Precedence: bulk
MIME-Version: 1.0

On Mon, 2024-05-06 at 11:38 +0200, Sebastian Andrzej Siewior wrote:
> On 2024-05-06 10:43:49 [+0200], Paolo Abeni wrote:
> > On Fri, 2024-05-03 at 20:25 +0200, Sebastian Andrzej Siewior wrote:
> > > Disabling bottoms halves acts as per-CPU BKL. On PREEMPT_RT code with=
in
> > > local_bh_disable() section remains preemtible. As a result high prior
> > > tasks (or threaded interrupts) will be blocked by lower-prio task (or
> > > threaded interrupts) which are long running which includes softirq
> > > sections.
> > >=20
> > > The proposed way out is to introduce explicit per-CPU locks for
> > > resources which are protected by local_bh_disable() and use those onl=
y
> > > on PREEMPT_RT so there is no additional overhead for !PREEMPT_RT buil=
ds.
> >=20
> > Let me rephrase to check I understood the plan correctly.
> >=20
> > The idea is to pair 'bare' local_bh_{disable,enable} with local lock
> > and late make local_bh_{disable,enable} no ops (on RT).
> >=20
> > With 'bare' I mean not followed by a spin_lock() - which is enough to
> > ensure mutual exclusion vs BH on RT build - am I correct?
>=20
> I might have I misunderstood your rephrase. But to make it clear:
> > $ git grep -p local_lock\( kernel/softirq.c
> > kernel/softirq.c=3Dvoid __local_bh_disable_ip(unsigned long ip, unsigne=
d int cnt)
> > kernel/softirq.c:                       local_lock(&softirq_ctrl.lock);
>=20
> this is what I want to remove. This is upstream RT only (not RT queue
> only). !RT builds are not affected by this change.

I was trying to describe the places that need the additional
local_lock(), but I think we are on the same page WRT the overall
semantic.

> >=20
> > Note that some callers use local_bh_disable(), no additional lock, and
> > there is no specific struct to protect, but enforce explicit
> > serialization vs bh to a bunch of operation, e.g.  the
> > local_bh_disable() in inet_twsk_purge().
> >=20
> > I guess such call site should be handled, too?
>=20
> Yes but I didn't find much. inet_twsk_purge() is the first item from my
> list. On RT spin_lock() vs spin_lock_bh() is the first item from my
> list. On RT spin_lock() vs spin_lock_bh() usage does not deadlock and
> could be mixed.
>=20
> The only resources that can be protected by disabling BH are per-CPU
> resources. Either explicit defined (such as napi_alloc_cache) or
> implicit by other means of per-CPU usage such as a CPU-bound timer,
> worker, =E2=80=A6. Protecting global variables by disabling BH is broken =
on SMP
> (see the CAN gw example) so I am not too worried about those.
> Unless you are aware of a category I did not think of.

I think sometimes the stack could call local_bh_enable() after a while
WRT the paired spin lock release, to enforce some serialization - alike
what inet_twsk_purge() is doing - but I can't point to any specific
line on top of my head.

A possible side-effect you should/could observe in the final tree is
more pressure on the process scheduler, as something alike:

local_bh_disable()

<spinlock lock unlock>

<again spinlock lock unlock>

local_bh_enable()

could results in more invocation of the scheduler, right?=20

Cheers,

Paolo