Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

Paul E. McKenney
2018-07-17T18:31:28Z

On Tue, Jul 17, 2018 at 09:19:15AM -0700, Linus Torvalds wrote:
> On Tue, Jul 17, 2018 at 7:45 AM Michael Ellerman <mpe@ellerman.id.au> wrote:
> >
> >
> > Interesting. I don't see anything as high as 18%, it's more spread out:
> >
> > 7.81% context_switch [kernel.kallsyms] [k] cgroup_rstat_updated
>
> Oh, see that's the difference.
>
> You're running in a non-root cgroup, I think.
>
> That also means that your scheduler overhead has way more spinlocks,
> and in particular, you have that
>
> raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);
> ..
> raw_spin_lock_irqsave(cpu_lock, flags);
>
> there too.
>
> So you have at least twice the spinlocks that my case had, and yes,
> the costs are way more spread out because your case has all that
> cgroup accounting too.
>
> That said, I don't understand the powerpc memory ordering. I thought
> the rules were "isync on lock, lwsync on unlock".
>
> That's what the AIX docs imply, at least.
>
> In particular, I find:
>
> "isync is not a memory barrier instruction, but the
> load-compare-conditional branch-isync sequence can provide this
> ordering property"
>
> so why are you doing "sync/lwsync", when it sounds like "isync/lwsync"
> (for lock/unlock) is the right thing and would already give memory
> barrier semantics?

The PowerPC guys will correct me if I miss something here...

The isync provides ordering roughly similar to lwsync, but nowhere near
as strong as sync, and it is sync that would be needed to cause lock
acquisition to provide full ordering. The reason for using lwsync instead
of isync is that the former proved to be faster on recent hardware.
The reason that the kernel still has the ability to instead generate
isync instructions is that some older PowerPC hardware does not provide
the lwsync instruction. If the hardware does support lwsync, the isync
instructions are overwritten with lwsync at boot time.

Thanx, Paul