Messages in this thread | | | Subject | Re: [RESEND RFC PATCH V3] sched: Improve scalability of select_idle_sibling using SMT balance | From | Steven Sistare <> | Date | Fri, 2 Feb 2018 15:51:25 -0500 |
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On 2/2/2018 2:58 PM, Peter Zijlstra wrote: > On Fri, Feb 02, 2018 at 12:36:47PM -0500, Steven Sistare wrote: >> On 2/2/2018 12:17 PM, Peter Zijlstra wrote: >>> On Fri, Feb 02, 2018 at 11:53:40AM -0500, Steven Sistare wrote: >>>>>> +static int select_idle_smt(struct task_struct *p, struct sched_group *sg) >>>>>> { >>>>>> + int i, rand_index, rand_cpu; >>>>>> + int this_cpu = smp_processor_id(); >>>>>> >>>>>> + rand_index = CPU_PSEUDO_RANDOM(this_cpu) % sg->group_weight; >>>>>> + rand_cpu = sg->cp_array[rand_index]; >>>>> >>>>> Right, so yuck.. I know why you need that, but that extra array and >>>>> dereference is the reason I never went there. >>>>> >>>>> How much difference does it really make vs the 'normal' wrapping search >>>>> from last CPU ? >>>>> >>>>> This really should be a separate patch with separate performance numbers >>>>> on. >>>> >>>> For the benefit of other readers, if we always search and choose starting from >>>> the first CPU in a core, then later searches will often need to traverse the first >>>> N busy CPU's to find the first idle CPU. Choosing a random starting point avoids >>>> such bias. It is probably a win for processors with 4 to 8 CPUs per core, and >>>> a slight but hopefully negligible loss for 2 CPUs per core, and I agree we need >>>> to see performance data for this as a separate patch to decide. We have SPARC >>>> systems with 8 CPUs per core. >>> >>> Which is why the current code already doesn't start from the first cpu >>> in the mask. We start at whatever CPU the task ran last on, which is >>> effectively 'random' if the system is busy. >>> >>> So how is a per-cpu rotor better than that? >> >> The current code is: >> for_each_cpu(cpu, cpu_smt_mask(target)) { >> >> For an 8-cpu/core processor, 8 values of target map to the same cpu_smt_mask. >> 8 different tasks will traverse the mask in the same order. > > Ooh, the SMT loop.. yes that can be improved. But look at the other > ones, they do: > > for_each_cpu_wrap(cpu, sched_domain_span(), target) > > so we look for an idle cpu in the LLC domain, and start iteration at > @target, which will (on average) be different for different CPUs, and > thus hopefully find different idle CPUs. > > You could simple change the SMT loop to something like: > > for_each_cpu_wrap(cpu, cpu_smt_mask(target), target) > > and see what that does.
Good idea - Steve
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