On Fri, Apr 07, 2017 at 01:30:11AM +1000, Nicholas Piggin wrote:
> On Thu, 6 Apr 2017 15:13:53 +0100
> Will Deacon wrote:
> > On Thu, Apr 06, 2017 at 10:59:58AM +1000, Nicholas Piggin wrote:
> > > Thanks for taking a look. The default spin primitives should just
> > > continue to do the right thin
On Fri, 7 Apr 2017 11:43:49 +0200
Peter Zijlstra wrote:
> On Thu, Apr 06, 2017 at 10:31:46AM -0700, Linus Torvalds wrote:
> > But maybe "monitor" is really cheap. I suspect it's microcoded,
> > though, which implies "no".
>
> On my IVB-EP (will also try on something newer):
>
> MONITOR
On Thu, Apr 06, 2017 at 10:31:46AM -0700, Linus Torvalds wrote:
> But maybe "monitor" is really cheap. I suspect it's microcoded,
> though, which implies "no".
On my IVB-EP (will also try on something newer):
MONITOR ~332 cycles
MWAIT ~224 cycles (C0, explicitly invalidated MONITOR)
So yes, ex
On Thu, 6 Apr 2017 12:41:52 -0700
Linus Torvalds wrote:
> On Thu, Apr 6, 2017 at 12:23 PM, Peter Zijlstra wrote:
> >
> > Something like so then. According to the SDM mwait is a no-op if we do
> > not execute monitor first. So this variant should get the first
> > iteration without expensive inst
On Thu, Apr 6, 2017 at 12:23 PM, Peter Zijlstra wrote:
>
> Something like so then. According to the SDM mwait is a no-op if we do
> not execute monitor first. So this variant should get the first
> iteration without expensive instructions.
No, the problem is that we *would* have executed a prior
On Thu, Apr 06, 2017 at 10:31:46AM -0700, Linus Torvalds wrote:
> And we'd probably want to make it even more strict, in that soem mwait
> implementations might simply not be very good for short waits.
Yeah, we need to find something that works; assuming its beneficial at
all on modern chips.
> >
On Thu, Apr 6, 2017 at 9:36 AM, Peter Zijlstra wrote:
>
> Something like the below, which is ugly (because I couldn't be bothered
> to resolve the header recursion and thus duplicates the monitor/mwait
> functions) and broken (because it hard assumes the hardware can do
> monitor/mwait).
Yeah, I
On Thu, Apr 06, 2017 at 08:16:19AM -0700, Linus Torvalds wrote:
> In theory x86 could use monitor/mwait for it too, in practice I think
> it tends to still be too high latency (because it was originally just
> designed for the idle loop). mwait got extended to actually be useful,
> but I'm not sur
On Thu, 6 Apr 2017 15:13:53 +0100
Will Deacon wrote:
> Hi Nick,
>
> On Thu, Apr 06, 2017 at 10:59:58AM +1000, Nicholas Piggin wrote:
> > On Wed, 05 Apr 2017 07:01:57 -0700 (PDT)
> > David Miller wrote:
> >
> > > From: Nicholas Piggin
> > > Date: Tue, 4 Apr 2017 13:02:33 +1000
> > >
> > >
On Thu, Apr 6, 2017 at 7:13 AM, Will Deacon wrote:
>
> We've wrapped this up in the arm64 code as __cmpwait, and we use that
> to build smp_cond_load_acquire. It would be nice to use the same machinery
> for the conditional spinning here, unless you anticipate that we're only
> going to be spinnin
Hi Nick,
On Thu, Apr 06, 2017 at 10:59:58AM +1000, Nicholas Piggin wrote:
> On Wed, 05 Apr 2017 07:01:57 -0700 (PDT)
> David Miller wrote:
>
> > From: Nicholas Piggin
> > Date: Tue, 4 Apr 2017 13:02:33 +1000
> >
> > > On Mon, 3 Apr 2017 17:43:05 -0700
> > > Linus Torvalds wrote:
> > >
> >
On Wed, 05 Apr 2017 07:01:57 -0700 (PDT)
David Miller wrote:
> From: Nicholas Piggin
> Date: Tue, 4 Apr 2017 13:02:33 +1000
>
> > On Mon, 3 Apr 2017 17:43:05 -0700
> > Linus Torvalds wrote:
> >
> >> But that depends on architectures having some pattern that we *can*
> >> abstract. Would som
From: Nicholas Piggin
Date: Tue, 4 Apr 2017 13:02:33 +1000
> On Mon, 3 Apr 2017 17:43:05 -0700
> Linus Torvalds wrote:
>
>> But that depends on architectures having some pattern that we *can*
>> abstract. Would some "begin/in-loop/end" pattern like the above be
>> sufficient?
>
> Yes. begin/in
On Tue, 4 Apr 2017 13:02:33 +1000
Nicholas Piggin wrote:
> On Mon, 3 Apr 2017 17:43:05 -0700
> Linus Torvalds wrote:
>
> > But that depends on architectures having some pattern that we *can*
> > abstract. Would some "begin/in-loop/end" pattern like the above be
> > sufficient?
>
> Yes. begi
On Mon, 3 Apr 2017 17:43:05 -0700
Linus Torvalds wrote:
> On Mon, Apr 3, 2017 at 4:50 PM, Nicholas Piggin wrote:
> > If you have any ideas, I'd be open to them.
>
> So the idea would be that maybe we can just make those things
> explicit. IOW, instead of having that magical looping construct
On Mon, Apr 3, 2017 at 4:50 PM, Nicholas Piggin wrote:
>
> POWER does not have an instruction like pause. We can only set current
> thread priority, and current implementations do something like allocate
> issue cycles to threads based on relative priorities. So there should
> be at least one or t
On Mon, 3 Apr 2017 08:31:30 -0700
Linus Torvalds wrote:
> On Mon, Apr 3, 2017 at 1:13 AM, Nicholas Piggin wrote:
> >
> > The loops have some restrictions on what can be used, but they are
> > intended to be small and simple so it's not generally a problem:
> > - Don't use cpu_relax.
> > - Don'
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