On Wed, 2014-11-05 at 11:06 -0600, Paul Clarke wrote: > Sorry it took me so long to get back to this... > > On 10/07/2014 09:52 PM, Michael Ellerman wrote: > > On Tue, 2014-07-10 at 19:13:24 UTC, Paul Clarke wrote: > >> This patch short-circuits the reset of the decrementer, exiting after > >> the decrementer reset, but before the housekeeping tasks if the only > >> need for the interrupt is simply to reset it. After this patch, > >> the latency spike was measured at about 150 nanoseconds. > > > Thanks for the excellent changelog. But this patch makes me a bit nervous :) > > > > Do you know where the latency is coming from? Is it primarily the irq work? > > Yes, it is all under irq_enter (measured at ~10us) and irq_exit (~12us).
Hmm, OK. I actually meant irq_work_run(). AIUI irq_enter/exit() are just state tracking, they shouldn't be actually running work. How are you measuring it? > > If so I'd prefer if we could move the short circuit into __timer_interrupt() > > itself. That way we'd still have the trace points usable, and it would > > hopefully result in less duplicated logic. > > But irq_enter and irq_exit are called in timer_interrupt, before > __timer_interrupt is called. I don't see how that helps. The time > spent in __timer_interrupt is minuscule by comparison. Right, it won't help if it's irq_enter() that is causing the delay. But I was assuming it was irq_work_run(). > Are you suggesting that irq_enter/exit be moved into __timer_interrupt > as well? (I'm not sure how that would impact the existing call to > __timer_interrupt from tick_broadcast_ipi_handler? And if there is no > impact, what's the point of separating timer_interrupt and > __timer_interrupt?) The point is __timer_interrupt() is called from tick_broadcast_ipi_handler(), which is called from smp_ipi_demux(), from icp_hv_ipi_action(), from __do_irq(), which has already done irq_enter() (and will do irq_exit()). cheers _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
