Re: [PATCH 1/3] clocksource/mips-gic-timer: Fix rcu_sched timeouts from multithreading

[Matt Redfearn]

On 19/10/17 10:09, Daniel Lezcano wrote:
> On 11/10/2017 16:01, Matt Redfearn wrote:
> > When the MIPS GIC clockevent code was written, it appears to have
> > inherited the 0x300 cycle min delta from the MIPS CPU timer driver. This
> > is suboptimal for two reasons.
> >
> > Firstly, the CPU timer counts once every other cycle (i.e. half the
> > clock rate). The GIC counts once per clock. Assuming that the GIC and
> > CPU share the same clock this means the GIC is counting twice as fast,
> > and so the min delta should be (at least) doubled. Fix this by doubling
> > the min delta to 0x600.
> >
> > Secondly, the fixed min delta ignores the fact that with MIPS
> > multithreading active, execution resource within a core is shared
> > between the hardware threads within that core. An inconvenienly timed
> > switch of executing thread within gic_next_event, between the read and
> > write of updated count, can result in the CPU writing an event in the
> > past, and subsequently not receiving a tick interrupt until the counter
> > wraps. This stalls the CPU from the RCU scheduler. Other CPUs detect
> > this and print rcu_sched timeout messages in  the kernel log. It can
> > lead to other issues as well if the CPU is holding locks or other
> > resources at the point at which it stalls. Fix this by scaling the min
> > delta for the timer based on the number of threads in the core
> > (smp_num_siblings). This accounts for the greater average runtime of
> > CPUs within a multithreading core.
> >
> > Signed-off-by: Matt Redfearn <matt.redfearn@xxxxxxxxxx>
> > Fixes: b695d8e6ad6f ("clocksource: mips-gic: Use clockevents_config_and_register")
> > Cc: <stable@xxxxxxxxxxxxxxx> # v3.19 +
> >
> > Signed-off-by: Matt Redfearn <matt.redfearn@xxxxxxxx>
> > ---
> Matt,
>
> I'm dropping your series.
>
> The first patch fails to compile because of the smp_num_siblings
> variable undefined when compile testing on another arch (probably a
> header is missing).
>
> The issue the patch address could be fixed in the time framework as
> stated by Thomas.
>
> As spotted by Thomas also, the local_irq_save() is not needed in the
> set_next_event() function, so the second patch is pointless and a patch
> removing those local_irq_save()/restore() would make more sense.
>
> The third patch does not longer apply after removing the two above.
>
>   -- Daniel

Hi Daniel,

Yep, makes sense that's fine - thanks!

Matt