On 19/10/2017 11:18, Thomas Gleixner wrote: > On Thu, 19 Oct 2017, Daniel Lezcano wrote: >> On 18/10/2017 22:34, Thomas Gleixner wrote: >>> On Wed, 11 Oct 2017, 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. >>> >>> I don't understand why this is not catched by the check at the end of the >>> next_event() function: >>> >>> res = ((int)(gic_read_count() - cnt) >= 0) ? -ETIME : 0; >>> >>> Btw, the local_irq_save() in this function is pointless as this function is >>> always called with interrupts disabled from the core code. >> >> Would it be worth to add some comment in include/linux/clockchips.h in >> the structure definition for the different callbacks to tell which ones >> are called with the irq disabled ? > > Yes. IIRC all callbacks are invoked with interrupts disabled. Care to check > that and whip up a patch? Sure, no problem. -- <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | <http://twitter.com/#!/linaroorg> Twitter | <http://www.linaro.org/linaro-blog/> Blog
