linux-kernel - Re: [PATCH] mutex: Speed up mutex_spin_on

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Date:	Fri, 10 Apr 2015 19:44:00 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jason Low <jason.low2@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mutex: Speed up mutex_spin_on_owner() by not taking the
 RCU lock


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> > No RCU overhead, and this is the access to owner->on_cpu:
> > 
> >   69:	49 8b 81 10 c0 ff ff 	mov    -0x3ff0(%r9),%rax
> > 
> > Totally untested and all that, I only built the mutex.o.
> > 
> > What do you think? Am I missing anything?
> 
> I suspect it is good, but let's take a look at Linus' summary of the code:
> 
>         rcu_read_lock();
>         while (sem->owner == owner) {
>                 if (!owner->on_cpu || need_resched())
>                         break;
>                 cpu_relax_lowlatency();
>         }
>         rcu_read_unlock();

Note that I patched the mutex case as a prototype, which is more 
commonly used than rwsem-xadd. But the rwsem case is similar as well.

> The cpu_relax_lowlatency() looks to have barrier() semantics, so the 
> sem->owner should get reloaded every time through the loop.  This is 
> needed, because otherwise the task structure could get freed and 
> reallocated as something else that happened to have the field at the 
> ->on_cpu offset always zero, resulting in an infinite loop.

So at least with the get_kernel(..., &owner->on_cpu) approach, the 
get_kernel() copy has barrier semantics as well (it's in assembly), so 
it will be reloaded in every iteration in a natural fashion.

Thanks,

	Ingo
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