linux-kernel - Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU from both process and interrupt context

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Date:   Tue, 6 Jun 2017 12:53:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, Paolo Bonzini <pbonzini@...hat.com>,
        kvm@...r.kernel.org, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH RFC tip/core/rcu 1/2] srcu: Allow use of Tiny/Tree SRCU
 from both process and interrupt context

On Mon, Jun 05, 2017 at 03:09:50PM -0700, Paul E. McKenney wrote:
> There would be a slowdown if 1) fast this_cpu_inc is not available and
> cannot be implemented (this usually means that atomic_inc has implicit
> memory barriers),

I don't get this.

How is per-cpu crud related to being strongly ordered?

this_cpu_ has 3 forms:

	x86:		single instruction
	arm64,s390:	preempt_disable()+atomic_op
	generic:	local_irq_save()+normal_op

Only s390 is TSO, arm64 is very much a weak arch.

>                   and 2) local_irq_save/restore is slower than disabling
> preemption.  The main architecture with these constraints is s390, which
> however is already paying the price in __srcu_read_unlock and has not
> complained.

IIRC only PPC (and hopefully soon x86) has a local_irq_save() that is as
fast as preempt_disable().

> A valid optimization on s390 would be to skip the smp_mb;
> AIUI, this_cpu_inc implies a memory barrier (!) due to its implementation.

You mean the s390 this_cpu_inc() in specific, right? Because
this_cpu_inc() in general does not imply any such thing.