linux-kernel - Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage

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Date:   Wed, 8 Aug 2018 10:27:00 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Joel Fernandes <joelaf@...gle.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Glexiner <tglx@...utronix.de>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>
Subject: Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and
 unify their usage

On Wed, 8 Aug 2018 06:00:41 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> I suppose that an srcu_read_lock_nmi() and srcu_read_unlock_nmi() could
> be added, which would do atomic ops on sp->sda->srcu_lock_count.  Not sure
> whether this would be fast enough to be useful, but easy to provide:
> 
> int __srcu_read_lock_nmi(struct srcu_struct *sp)  /* UNTESTED. */
> {
> 	int idx;
> 
> 	idx = READ_ONCE(sp->srcu_idx) & 0x1;
> 	atomic_inc(&sp->sda->srcu_lock_count[idx]);
> 	smp_mb__after_atomic(); /* B */  /* Avoid leaking critical section. */
> 	return idx;
> }
> 
> void __srcu_read_unlock_nmi(struct srcu_struct *sp, int idx)
> {
> 	smp_mb__before_atomic(); /* C */  /* Avoid leaking critical section. */
> 	atomic_inc(&sp->sda->srcu_unlock_count[idx]);
> }
> 
> With appropriate adjustments to also allow Tiny RCU to also work.
> 
> Note that you have to use _nmi() everywhere, not just in NMI handlers.
> In fact, the NMI handlers are the one place you -don't- need to use
> _nmi(), strangely enough.
> 
> Might be worth a try -- smp_mb__{before,after}_atomic() is a no-op on
> some architectures, for example.

Note this would kill the performance that srcu gives that Joel wants.
Switching from a this_cpu_inc() to a atomic_inc() would be a huge
impact.

There's also a local_inc() if you are using per cpu pointers, that is
suppose to guarantee atomicity for single cpu operations. That's what
the ftrace ring buffer uses.

-- Steve