Re: [patch 54/55] timekeeping: Provide fast and NMI safe access to CLOCK_MONOTONIC[_RAW]

[Mathieu Desnoyers]

----- Original Message -----
> From: "Thomas Gleixner" <tglx@xxxxxxxxxxxxx>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: "LKML" <linux-kernel@xxxxxxxxxxxxxxx>, "John Stultz" <john.stultz@xxxxxxxxxx>, "Peter Zijlstra"
> <peterz@xxxxxxxxxxxxx>, "Steven Rostedt" <rostedt@xxxxxxxxxxx>
> Sent: Saturday, July 12, 2014 4:11:17 AM
> Subject: Re: [patch 54/55] timekeeping: Provide fast and NMI safe access to CLOCK_MONOTONIC[_RAW]
> 
> On Fri, 11 Jul 2014, Mathieu Desnoyers wrote:
> 
> > ----- Original Message -----
> > > From: "Thomas Gleixner" <tglx@xxxxxxxxxxxxx>
> > > To: "LKML" <linux-kernel@xxxxxxxxxxxxxxx>
> > > Cc: "John Stultz" <john.stultz@xxxxxxxxxx>, "Peter Zijlstra"
> > > <peterz@xxxxxxxxxxxxx>, "Steven Rostedt"
> > > <rostedt@xxxxxxxxxxx>, "Mathieu Desnoyers"
> > > <mathieu.desnoyers@xxxxxxxxxxxx>
> > > Sent: Friday, July 11, 2014 9:45:19 AM
> > > Subject: [patch 54/55] timekeeping: Provide fast and NMI safe access to
> > > CLOCK_MONOTONIC[_RAW]
> > > On the update side:
> > > 
> > >   tkf->seq++;
> > >   smp_wmb();
> > >   update(tkf->base[0], tk;
> > 
> > missing ")"
> 
> :)
>  
> > Any reason why the updater wouldn't do:
> > 
> > tkf->seq++;
> > smp_wmb();
> > update(tkf->base[1 - (tkf->seq & 0x01)], tk);
> > 
> > instead of updating both array entries each time ?
> 
>   base[0];   	      <- Current active
>   seq++;	      -> Makes base[1] the active one for readers
>   update(base[0]);
> 
> So readers are always one update cycle behind. Probably not an issue
> most of the time, but think about fast wrapping clocksources and
> NOHZ....

Ah, yep, got it.

So instead of calling update() twice per update, could we do
the following instead ?

tkf->seq++;
smp_wmb();
update(tkf->base[0], tk);
tkf->seq++;
smp_wmb();
memcpy(tkf->base[1], tkf->base[0], sizeof(tkf->base[1]);

just in case "update" would happen to be expensive for
some clock implementation.

Thanks,

Mathieu


> 
> Thanks,
> 
> 	tglx
> 
> 
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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