On 10/08/2014 08:22 AM, Michael Ellerman wrote: > On Tue, 2014-07-10 at 19:13:24 UTC, Paul Clarke wrote: >> The POWER ISA defines an always-running decrementer which can be used >> to schedule interrupts after a certain time interval has elapsed. >> The decrementer counts down at the same frequency as the Time Base, >> which is 512 MHz. The maximum value of the decrementer is 0x7fffffff. >> This works out to a maximum interval of about 4.19 seconds. >> >> If a larger interval is desired, the kernel will set the decrementer >> to its maximum value and reset it after it expires (underflows) >> a sufficient number of times until the desired interval has elapsed. >> >> The negative effect of this is that an unwanted latency spike will >> impact normal processing at most every 4.19 seconds. On an IBM >> POWER8-based system, this spike was measured at about 25-30 >> microseconds, much of which was basic, opportunistic housekeeping >> tasks that could otherwise have waited. >> >> 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. > > Hi Paul, > > 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? > > 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.
I agree, this is perhaps the better approach. Regards Preeti U Murthy > > cheers > _______________________________________________ > Linuxppc-dev mailing list > Linuxppc-dev@lists.ozlabs.org > https://lists.ozlabs.org/listinfo/linuxppc-dev > _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
