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 ? -- <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
