Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1251438

Re: Q: schedule() and implied barriers on arm64

Show all headers | View raw

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, Oct 19, 2015 at 04:21:08PM +0100, Catalin Marinas wrote:
> > On Mon, Oct 19, 2015 at 09:06:05AM +0200, Ingo Molnar wrote:
> > > * Peter Zijlstra <peterz@infradead.org> wrote:
> > > 
> > > > In any case, its all moot now, since Paul no longer requires schedule() to imply 
> > > > a full barrier.
> > > >
> > > > [...]
> > > 
> > > Nevertheless from a least-surprise POV it might be worth guaranteeing it, 
> > > because I bet there's tons of code that assumes that schedule() is a heavy 
> > > operation and it's such an easy mistake to make. Since we are so close to 
> > > having that guarantee, we might as well codify it?
> > 
> > FWIW, the arm64 __switch_to() has a heavy barrier (DSB) but the reason for 
> > this was to cope with potentially interrupted cache or TLB maintenance (which 
> > require a DSB on the same CPU) and thread migration to another CPU.
> 
> Right, but there's a path through schedule() that does not pass through 
> __switch_to(); when we pick the current task as the most eligible task and next 
> == prev.
> 
> In that case there really only is the wmb, a spin lock, an atomic op and a spin 
> unlock (and a whole bunch of 'normal' code of course).

Yeah, so my concern is that this is a rare race that might be 'surprising' for 
developers relying on various schedule() constructs. Especially as it's a full 
barrier on x86 (the most prominent SMP platform at the moment) there's a real 
danger of hard to debug bugs creeping to other architectures.

So I think we should just do the small step of making it a full barrier everywhere 
- it's very close to it in any case, and it shouldn't really matter for 
performance. Agreed?


Thanks,

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Re: Q: schedule() and implied barriers on arm64 Ingo Molnar <mingo@kernel.org> - 2015-10-19 09:10 +0200
  Re: Q: schedule() and implied barriers on arm64 Peter Zijlstra <peterz@infradead.org> - 2015-10-19 11:10 +0200
  Re: Q: schedule() and implied barriers on arm64 Catalin Marinas <catalin.marinas@arm.com> - 2015-10-19 17:30 +0200
    Re: Q: schedule() and implied barriers on arm64 Peter Zijlstra <peterz@infradead.org> - 2015-10-19 18:30 +0200
      Re: Q: schedule() and implied barriers on arm64 Ingo Molnar <mingo@kernel.org> - 2015-10-20 10:40 +0200
      Re: Q: schedule() and implied barriers on arm64 Will Deacon <will.deacon@arm.com> - 2015-10-27 17:30 +0100
        Re: Q: schedule() and implied barriers on arm64 Peter Zijlstra <peterz@infradead.org> - 2015-10-28 01:20 +0100
          Re: Q: schedule() and implied barriers on arm64 Will Deacon <will.deacon@arm.com> - 2015-10-28 11:50 +0100

csiph-web