linux-kernel - Re: [PATCH 11/11] x86,rcu: use percpu rcu_preempt

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Date:   Fri, 1 Nov 2019 07:30:36 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andy Lutomirski <luto@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Babu Moger <Babu.Moger@....com>,
        Rik van Riel <riel@...riel.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Jann Horn <jannh@...gle.com>,
        David Windsor <dwindsor@...il.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Yuyang Du <duyuyang@...il.com>,
        Richard Guy Briggs <rgb@...hat.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Dmitry V. Levin" <ldv@...linux.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 11/11] x86,rcu: use percpu rcu_preempt_depth

On Fri, Nov 01, 2019 at 02:13:15PM +0100, Peter Zijlstra wrote:
> On Fri, Nov 01, 2019 at 05:58:16AM -0700, Paul E. McKenney wrote:
> > On Thu, Oct 31, 2019 at 10:08:06AM +0000, Lai Jiangshan wrote:
> > > +/* We mask the RCU_NEED_SPECIAL bit so that it return real depth */
> > > +static __always_inline int rcu_preempt_depth(void)
> > > +{
> > > +	return raw_cpu_read_4(__rcu_preempt_depth) & ~RCU_NEED_SPECIAL;
> > 
> > Why not raw_cpu_generic_read()?
> > 
> > OK, OK, I get that raw_cpu_read_4() translates directly into an "mov"
> > instruction on x86, but given that x86 percpu_from_op() is able to
> > adjust based on operand size, why doesn't something like raw_cpu_read()
> > also have an x86-specific definition that adjusts based on operand size?
> 
> The reason for preempt.h was header recursion hell.

Fair enough, being as that is also the reason for _rcu_read_lock()
not being inlined.  :-/

> > > +}
> > > +
> > > +static __always_inline void rcu_preempt_depth_set(int pc)
> > > +{
> > > +	int old, new;
> > > +
> > > +	do {
> > > +		old = raw_cpu_read_4(__rcu_preempt_depth);
> > > +		new = (old & RCU_NEED_SPECIAL) |
> > > +			(pc & ~RCU_NEED_SPECIAL);
> > > +	} while (raw_cpu_cmpxchg_4(__rcu_preempt_depth, old, new) != old);
> > 
> > Ummm...
> > 
> > OK, as you know, I have long wanted _rcu_read_lock() to be inlineable.
> > But are you -sure- that an x86 cmpxchg is faster than a function call
> > and return?  I have strong doubts on that score.
> 
> This is a regular CMPXCHG instruction, not a LOCK prefixed one, and that
> should make all the difference

Yes, understood, but this is also adding some arithmetic, a comparison,
and a conditional branch.  Are you -sure- that this is cheaper than
an unconditional call and return?

> > Plus multiplying the x86-specific code by 26 doesn't look good.
> > 
> > And the RCU read-side nesting depth really is a per-task thing.  Copying
> > it to and from the task at context-switch time might make sense if we
> > had a serious optimization, but it does not appear that we do.
> > 
> > You original patch some years back, ill-received though it was at the
> > time, is looking rather good by comparison.  Plus it did not require
> > architecture-specific code!
> 
> Right, so the per-cpu preempt_count code relies on the preempt_count
> being invariant over context switches. That means we never have to
> save/restore the thing.
> 
> For (preemptible) rcu, this is 'obviously' not the case.
> 
> That said, I've not looked over this patch series, I only got 1 actual
> patch, not the whole series, and I've not had time to go dig out the
> rest..

I have taken a couple of the earlier patches in the series.

Perhaps inlining these things is instead a job for the long anticipated
GCC LTO?  ;-)

							Thanx, Paul