Messages in this thread | | | Date | Mon, 5 Jan 2009 16:36:21 -0600 | From | Dimitri Sivanich <> | Subject | Re: 2.6.27.8 scheduler bug - threads not being scheduled for long periods |
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On Mon, Jan 05, 2009 at 03:54:29PM -0600, Dimitri Sivanich wrote: > On Mon, Jan 05, 2009 at 09:55:58PM +0100, Peter Zijlstra wrote: > > On Mon, 2009-01-05 at 11:56 -0600, Dimitri Sivanich wrote: > > > One place we've found this happens is in update_curr(), which calculates a > > > delta_exec value as follows: > > > delta_exec = (unsigned long)(now - curr->exec_start); > > > > > > Sometimes this value will be very large, as 'now' (the rq clock time) will > > > be less than 'exec_start'. When this happens, __update_curr() will > > > calculate a delta_exec_weighted based on this large value and add it to the > > > thread's vruntime: > > > curr->vruntime += delta_exec_weighted; > > > > So you're saying your rq->clock = sched_clock_cpu(cpu) = sched_clock() > > [on ia64] goes backwards? > > > > If so, then that's an architecture bug, sched_clock() must never be seen > > to go backwards! > > Actually, sched_clock() should not go backwards on any one cpu, but the readings will be different between cpus. > > Also, we noticed the following code is being used for sched_clock_cpu(): > u64 sched_clock_cpu(int cpu) > { > if (unlikely(!sched_clock_running)) > return 0; > return sched_clock(); > } > > and is called when smp_processor_id() != cpu.
And sure enough, the rq->clock is sometimes going backwards.
The comment for sched_clock() in arch/ia64/kernel/head.S: * Return a CPU-local timestamp in nano-seconds. This timestamp is * NOT synchronized across CPUs its return value must never be * compared against the values returned on another CPU. The usage in * kernel/sched.c ensures that.
We will try this with CONFIG_HAVE_UNSTABLE_SCHED_CLOCKS.
Thanks for the heads up!
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