Re: livelock with hrtimer cpu_base->lock

[Sodagudi Prasad]

On 2018-10-10 09:49, Will Deacon wrote:
> Hi Prasad,
>
> On Tue, Oct 09, 2018 at 01:56:14PM -0700, Sodagudi Prasad wrote:
> > This is regarding - thread "try to fix contention between 
> > expire_timers and
> > try_to_del_timer_sync".
> > https://lkml.org/lkml/2017/7/28/172
> >
> > I think this live lockup issue was discussed earlier but the final set 
> > of
> > changes were not concluded.
>
> Well we basically need a way to pick a value for 
> CPU_RELAX_WFE_THRESHOLD.
> Do you have any ideas? It could be determined at runtime if necessary.
Hi Will,

Please share what are values need to be tried for 
CPU_RELAX_WFE_THRESHOLD.

 It would be great if it can be determined from runtime.  Please let me 
know
if any testing need to be done with dynamic detection patch.

-Thanks, Prasad

> > I would like to check whether you have new updates on this issue or 
> > not.
> > This problem is observed with 4.14 .64 stable kernel too.
> > We see this problem 2 times in overnight testing.
> >
> > I have to add the following code to avoid live lock. I am thinking 
> > that
> > fixing this at the cpu_relax() level.
> >
> > +++ b/kernel/time/hrtimer.c
> > @@ -52,6 +52,7 @@
> >  #include <linux/timer.h>
> >  #include <linux/freezer.h>
> >  #include <linux/compat.h>
> > +#include <linux/delay.h>
> >
> >  #include <linux/uaccess.h>
> >
> > @@ -152,6 +153,7 @@ struct hrtimer_clock_base *lock_hrtimer_base(const
> > struct hrtimer *timer,
> > raw_spin_unlock_irqrestore(&base->cpu_base->lock, *flags);
> >                 }
> >                 cpu_relax();
> > +               udelay(1);
> >         }
> >  }
> >
> > @@ -1067,6 +1069,7 @@ int hrtimer_cancel(struct hrtimer *timer)
> >                 if (ret >= 0)
> >                         return ret;
> >                 cpu_relax();
> > +               udelay(1);
> >         }
> >  }
> >  EXPORT_SYMBOL_GPL(hrtimer_cancel);
>
> This is just another bodge and likely to hurt in places where 1us is
> excessive because there isn't contention.
>
> Will

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