linux-kernel - [RFC 0/3] Revert SRCU from tracepoint infrastructure

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Message-Id: <20200207205656.61938-1-joel@joelfernandes.org>
Date:   Fri,  7 Feb 2020 15:56:53 -0500
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Ingo Molnar <mingo@...hat.com>,
        Richard Fontana <rfontana@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: [RFC 0/3] Revert SRCU from tracepoint infrastructure

Hi,
These patches remove SRCU usage from tracepoints. The reason for proposing the
reverts is because the whole point of SRCU was to avoid having to call
rcu_irq_enter_irqson(). However this was added back in 865e63b04e9b2 ("tracing:
Add back in rcu_irq_enter/exit_irqson() for rcuidle tracepoints") because perf
was breaking..

Further it occurs to me that, by using SRCU for tracepoints, we forgot that RCU
is not really watching the tracepoint callbacks. This means that anyone doing
preempt_disable() in their tracepoint callback, and expecting RCU to listen to
them is in for a big surprise. When RCU is not watching, it does not care about
preempt-disable sections on CPUs as you can see in the forced-quiescent state loop.

Since SRCU is not providing any benefit because of 865e63b04e9b2 anyway, let us
revert SRCU tracepoint code to maintain the sanity of potential
tracepoint callback registerers.

Joel Fernandes (Google) (3):
Revert "tracepoint: Use __idx instead of idx in DO_TRACE macro to make
it unique"
Revert "tracing: Add back in rcu_irq_enter/exit_irqson() for rcuidle
tracepoints"
Revert "tracepoint: Make rcuidle tracepoint callers use SRCU"

include/linux/tracepoint.h | 40 ++++++--------------------------------
kernel/tracepoint.c        | 10 +---------
2 files changed, 7 insertions(+), 43 deletions(-)

--
2.25.0.341.g760bfbb309-goog