Re: [RFC][PATCH 01/13] sched/deadline: Impose global limits on sched_attr::sched_period

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Sep 04, 2019 at 03:11:10PM +0100, Will Deacon wrote:
> Hi Joel,
> 
> On Wed, Sep 04, 2019 at 09:24:18AM -0400, Joel Fernandes wrote:
> > On Wed, Sep 04, 2019 at 01:30:38PM +0200, Peter Zijlstra wrote:
> > > On Wed, Sep 04, 2019 at 06:16:16AM -0400, Steven Rostedt wrote:
> > > > On Mon, 2 Sep 2019 11:16:23 +0200
> > > > Peter Zijlstra <peterz@infradead.org> wrote:
> > > > 
> > > > > in sched_dl_period_handler(). And do:
> > > > > 
> > > > > +	preempt_disable();
> > > > > 	max = (u64)READ_ONCE(sysctl_sched_dl_period_max) * NSEC_PER_USEC;
> > > > > 	min = (u64)READ_ONCE(sysctl_sched_dl_period_min) * NSEC_PER_USEC;
> > > > > +	preempt_enable();
> > > > 
> > > > Hmm, I'm curious. Doesn't the preempt_disable/enable() also add
> > > > compiler barriers which would remove the need for the READ_ONCE()s here?
> > > 
> > > They do add compiler barriers; but they do not avoid the compiler
> > > tearing stuff up.
> > 
> > Neither does WRITE_ONCE() on some possibly buggy but currently circulating
> > compilers :(
> 
> Hmm. The example above is using READ_ONCE, which is a different kettle of
> frogs.

True. But, I equally worry about all *-tearing frog kettles ;-)

> > As Will said in:
> > https://lore.kernel.org/lkml/20190821103200.kpufwtviqhpbuv2n@willie-the-truck/
> > 
> > void bar(u64 *x)
> > {
> > 	*(volatile u64 *)x = 0xabcdef10abcdef10;
> > }
> > 
> > gives:
> > 
> > bar:
> > 	mov	w1, 61200
> > 	movk	w1, 0xabcd, lsl 16
> > 	str	w1, [x0]
> > 	str	w1, [x0, 4]
> > 	ret
> > 
> > Speaking of which, Will, is there a plan to have compiler folks address this
> > tearing issue and are bugs filed somewhere? I believe aarch64 gcc is buggy,
> > and clang is better but is still buggy?
> 
> Well, it depends on your point of view. Personally, I think tearing a
> volatile access (e.g. WRITE_ONCE) is buggy and it seems as though the GCC
> developers agree:
> 
> https://gcc.gnu.org/ml/gcc-patches/2019-08/msg01500.html
> 
> so it's likely this will be fixed for AArch64 GCC. I couldn't persuade
> clang to break the volatile case, so think we're good there too.

Glad to know that GCC folks are looking into the issue.

Sorry if this is getting a bit off-topic. Also does the aarch64 clang doing
the "memset folding" issue, also need to be looked into?
You had mentioned it in the same thread:
https://lore.kernel.org/lkml/20190821103200.kpufwtviqhpbuv2n@willie-the-truck/
Or, does WRITE_ONCE() resolve such memset store-merging?

> For the non-volatile case, I don't actually consider it to be a bug,
> although I sympathise with the desire to avoid a retrospective tree-wide
> sweep adding random WRITE_ONCE invocations to stores that look like they
> might be concurrent. In other words, I think I'd suggest:
> 
>   * The use of WRITE_ONCE in new code (probably with a comment justifying it)
>   * The introduction of WRITE_ONCE to existing code where it can be shown to
>     be fixing a real bug (e.g. by demonstrating that a compiler actually
>     gets it wrong)
> 
> For the /vast/ majority of cases, the compiler will do the right thing
> even without WRITE_ONCE, simply because that's going to be the most
> performant choice as well.

Thanks for the thoughts. They seem to be reasonable to me.

thanks,

 - Joel