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? Thanks, tglx
