Messages in this thread | | | From | "Chris Mason" <> | Subject | Re: [PATCH] fix scheduler regression from "sched/fair: Rework load_balance()" | Date | Mon, 26 Oct 2020 08:45:27 -0400 |
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On 26 Oct 2020, at 4:39, Vincent Guittot wrote:
> Hi Chris > > On Sat, 24 Oct 2020 at 01:49, Chris Mason <clm@fb.com> wrote: >> >> Hi everyone, >> >> We’re validating a new kernel in the fleet, and compared with v5.2, > > Which version are you using ? > several improvements have been added since v5.5 and the rework of > load_balance
We’re validating v5.6, but all of the numbers referenced in this patch are against v5.9. I usually try to back port my way to victory on this kind of thing, but mainline seems to behave exactly the same as 0b0695f2b34a wrt this benchmark.
> >> performance is ~2-3% lower for some of our workloads. After some >> digging, Johannes found that our involuntary context switch rate was >> ~2x >> higher, and we were leaving a CPU idle a higher percentage of the >> time, >> even though the workload was trying to saturate the system. >> >> We were able to reproduce the problem with schbench, and Johannes >> bisected down to: >> >> commit 0b0695f2b34a4afa3f6e9aa1ff0e5336d8dad912 >> Author: Vincent Guittot <vincent.guittot@linaro.org> >> Date: Fri Oct 18 15:26:31 2019 +0200 >> >> sched/fair: Rework load_balance() >> >> Our working theory is the load balancing changes are leaving >> processes >> behind busy CPUs instead of moving them onto idle ones. I made a few >> schbench modifications to make this easier to demonstrate: >> >> https://git.kernel.org/pub/scm/linux/kernel/git/mason/schbench.git/ >> >> My VM has 40 cpus (20 cores, 2 threads per core), and my schbench >> command line is: > > What is the topology ? are they all part of the same LLC ?
We’ve seen the regression on both single socket and dual socket bare metal intel systems. On the VM I reproduced with, I saw similar latencies with and without siblings configured into the topology.
-chris
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